Need more info...what exactly do you mean that AT&T can't activate your iphone? Do you have an account with AT&T? Do you have an activated sim from AT&T with that account info on it? Have you connected your phone to itunes with the proper activated sim in the phone?
I dropped AT&T in Nov over service issues and moved to Verizon. Droid is a disaster so I want to turn my Iphone back on. AT&T says I have to go to a store, but I turned it on originally through Itunes. Can't access that feature now.
Then why don't you do that? You most likely will have to re-establish your account, credit check, get a sim card, etc. That's why they told you to go to a store. You can't get a sim card from itunes or Apple to re-establish your service.