Enabling InnoDB on an existing MySQL production server
InnoDB should already available. You can check with

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


or

mysql&gt; show variables like 'have_innodb';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| have_innodb   | YES   |
+---------------+-------+
1 row in set (0.00 sec)

mysql&gt;


Make sure skip-innodb is not in /etc/my.cnf

Since you are using the InnoDB Plugin, you need to activate new features to greatly enhance InnoDB Performance. For example, did you know there are new options for making InnoDB use multiple CPUs?  See my past posts on this:


May 26, 2011 : About single threaded versus multithreaded databases performance
Sep 12, 2011 : Possible to make MySQL use more than one core?
Sep 20, 2011 : Multi cores and MySQL Performance


This is important because there are occasions when InnoDB in older versions of MySQL is faster than newer versions. I wrote about this as well:


Jun 19, 2011 : How do I properly perform a MySQL bake-off?
Oct 05, 2011 : Query runs a long time in some newer MySQL versions
Nov 24, 2011 : Why mysql 5.5 slower than 5.1 (linux,using mysqlslap)


Additional Reference from MySQL Performance Blog : http://www.mysqlperformanceblog.com/2011/10/10/mysql-versions-shootout/