There's a dedicated US number which you'll find on the Challenge thread on the thread on the AA board which I strongly advise reading. http://www.flyertalk.com/forum/ameri...atinum-64.html

You just call and they enrol you from the date you want to start it. There is a fee but I think there's a discount at the moment. As you will no doubt have realised, there is a significant bonus if you start it after 16 June (the fee is a little higher though) so if you can hold off until after that it will pay you in the long run.

One thing that I don't think has been mentioned thus far is that it is common for them to instantly upgrade your status to Gold while you do the Platinum challenge, especially if you already have status with another airline. They usually offer it without prompting but well worth asking about if they don't.