Seems like you have updated your baseband to 05.15.04 which is still unlockable. If you have baseband 05.14.02 or 05.15.04 then to unlock, you have to install ipad basebad (you will loose GPS functionality ...first read the concequences in the link as well). 

http://www.iclarified.com/entry/index.php?enid=12949
You can get link to download everything you need.