The menu pretty well leads you down the path. The key is to make sure that you start on MENU -> 2:SCAN OPTION -> 2:TRUNK before you start trying to do anything. Once you pick the frequency bank you are working with, the next screen gives you 11 choices, you need to pick 3:TALK GROUP for entering the TG's. Of course you must have already chosen the type under 1:TRUNK TYPE so that the proper TG's can be entered.
Hope this helps.