Unable to change engine to MyISAM in MySQL
MySQL 5.6.10 is quite new to me as well, so bear with me on this answer

I installed MySQL 5.6.10 on a Linux VM

I added default-storage-engine=MyISAM to my.cnf and restarted MySQL

It went from this:

mysql&gt; show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                        | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| MyISAM             | YES     | MyISAM storage engine                                          | NO           | NO   | NO         |
| CSV                | YES     | CSV storage engine                                             | NO           | NO   | NO         |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |
| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |
| InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |
| FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)

to 

mysql&gt; show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                        | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| MyISAM             | DEFAULT | MyISAM storage engine                                          | NO           | NO   | NO         |
| CSV                | YES     | CSV storage engine                                             | NO           | NO   | NO         |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |
| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |
| InnoDB             | YES     | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |
| FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)


No problem.

If you placed

default-storage-engine=MyISAM


in my.cnf, restarted MySQL and still the default is InnoDB, then there is only one explanation: You placed default-storage-engine=MyISAM in the wrong part of my.cnf

I once caught a mistake like that in a ServerFault question : Set group_concat_max_len permanently (MySQL config)

Make sure the option is under the [mysqld] section of my.cnf

[mysqld]
default-storage-engine=MyISAM


SUGGESTION

I think MyISAM in MySQL 5.6.10 is run as a plugin. Check your error logs for any plugin errors.

Also, do this:

In one SSH session, tail -f /var/log/mysqld.log

In another SSH session, service mysql restart

Look for any plugin messages associated with MyISAM. For example, note that when I ran a shutdown I saw these messages:

2013-02-26 15:20:13 1725 [Note] InnoDB: Starting shutdown...
2013-02-26 15:20:15 1725 [Note] InnoDB: Shutdown completed; log sequence number 1600309
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'PERFORMANCE_SCHEMA'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'CSV'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'MEMORY'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'MRG_MYISAM'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'MyISAM'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'sha256_password'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'mysql_old_password'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'mysql_native_password'
2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'binlog'
2013-02-26 15:20:15 1725 [Note] /usr/sbin/mysqld: Shutdown complete

130226 15:20:15 mysqld_safe mysqld from pid file /var/lib/mysql/chtn-web2v.pid ended


Note that MyISAM is a plugin. If you do not see anything like

2013-02-26 15:20:15 1725 [Note] Shutting down plugin 'MyISAM'


your MyISAM plugin may have an issue with the Mac OS X version of MySQL.