Call Tele-charge and ask for Customer Service.
If you have the tickets in hand, be prepared to mail them back. They will give you the address. Credit will not be given until the tix are returned.
If they retained physical possession of the tix to be Held At Box Office (HABO-Isn't that cute?) your credit will be automatic. In either case you will get a letter or a phone call to this effect.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher