I know this is probably too late for the OP but I have spent hours searching for how to enable the federated engine on a yum mysql build so I thought I should put the answer here for future googlers:
Distro is Fedora 14, MySQL is a standard yum mysql-server (& mysql-devel, not sure if that is relevant for this federated engine install tho') install v5.1.55.
To enable the federated engine:
Check the file 'ha_federated.so' exists. In the distro I am using it is in '/usr/lib/mysql/plugin', if it does not exist anywhere then this fix is not for you.
Start your mysql shell with a user with insert access to the mysql.plugin table.
Enter "install plugin federated soname 'ha_federated.so';" (without the double quotes)
The message "Query OK, 1 warning", should display. "show warnings" should show a warning "1123 ...plugin is disabled", this is expected.
To check if the plugin has been installed run "select * from mysql.plugin"
Exit the MySQL shell.
Edit "my.cnf" (generally found in /etc) and insert (or uncomment) the line that contains the single word "federated"
Stop & restart the mysqld service.
Fire up the shell and create a table using the federated engine, if the above has worked then "show create table newtablename" should show that the engine is using the federated engine.