Why would one use the Innodb Plugin instead of the builtin Innodb in Mysql 5.1? (GPL)
The InnoDB plugin came into existence in MySQL 5.1 since version 5.1.38.

Many new features in the plugin allow you to


make InnoDB access more CPUs

innodb_read_io_threads
innodb_write_io_threads

scale up InnoDB to match the IOPs of your disk (innodb_io_capacity)


You are better off upgrading to MySQL 5.6 which has surpassed MySQL 5.1/5.5 because in  5.6


you can dump the buffer pool to disk on shutdown and reload it on startup
you can segment the buffer pool into buffer pool instances to help prevent swapping if the buffer pool is more than 50% of installed RAM


I would suggest you upgrade to MySQL 5.6 instead of dealing with enabling the old 5.1 plugin