From official reference
Full-text indexes can be used only with MyISAM tables. (In MySQL 5.6 and up, they can also be used with InnoDB tables.) Full-text indexes can be created only for CHAR, VARCHAR, or TEXT columns.
https://dev.mysql.com/doc/refman/5.5/en/fulltext-search.html
InnoDB with MySQL 5.5 does not support Full-text indexes.