Called T-Mobile regular customer services and person who answered confirmed there is a 12% AAA discount (without even mentioning the advert, just asking if there was a AAA member discount). I then got put through to "migration services" and spoke to the person there. Gave my T-Mobile account details, AAA card number and the advert promotion code (bottom right of the newsletter) TM/6531/09.
The represeantiave said it would take 30-45 days to verify with AAA that I am a member then the monthly discount should appear on my T-Mobile bill as an "adjustment". I didn't have to sign up for a new 1 or 2 year contract and I was cautioned it would run for as long as AAA were doing the promotion with T-Mobile.