InnoDB missing from MySQL
Since you have done the following:

SHOW VARIABLES LIKE 'innodb%';
No InnoDB-related variables exist in my.cnf

and nothing comes up, please place this in my.cnf
[mysqld]
innodb

and start mysql and see if it comes up
UPDATE 2012-05-15 15:59 EDT
Since it fails to start up, you may need uninstall MySQL 5.5.24 and go back to MySQL 5.5.23 or whichever version you were last running with.
UPDATE 2012-05-15 16:06 EDT
Please run tail -30 on the mysql error log and paste that in the question.f it is not configured, go to the data folder and do ls -l *.err to find it.
UPDATE 2012-05-15 17:54 EDT
Based on the display you pasted in, you evidently had MySQL 5.1 running before. It  just so happens that MySQL 5.1 has a table called mysql.plugin. It may have been populated before. It probably had something concerning the InnoDB plugin. MySQL 5.1.38 introduced the InnoDB plugin which has new features that are now native to MySQL 5.5's InnoDB. In other words, MySQL 5.5 does not need any plugin to run the new InnoDB.
Please run this query:
SELECT COUNT(1) FROM mysql.plugin;

If you get 1, that's probably the MySQL 5.1 InnoDB Plugin interfering. Try moving it over like this:
CREATE TABLE mysql.plugin_new LIKE mysql.plugin;
ALTER TABLE mysql.plugin RENAME mysql.plugin_old;
ALTER TABLE mysql.plugin_new RENAME mysql.plugin;

Afterwards, please restart mysql and see if InnoDB comes back