starting in august the new draft for the G I bill in at that time all old recruits will again be asked if they want to take it ... as far as i know they are going to give you the choice again or they will automatically inroll you...even though its still in the works you can start doing the research on it from what i hear its even better than before
talk to your S-1 to start the paper work. The S-1 NCOIC/OIC have full access to your records. if they can't do it they will give you the right paper work to take to your post PSB.
If you are trying to sign up for the Montgomery GI Bill it is too late. You must start the deductions for that benefit when you are in basic training.
If you are talking about the Post 9-11 GI Bill you don't have to sign up for that, enrollment is automatic, and benefits can be distributed once you have at least 24 months of honorable service.