Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)

Did you log in to your BAEC account and specify (from the drop-down window) that you wanted to pay with Avios before searching for the AA flight?

BA does not have local traffic rights for inter-U.S. travel, but that's irrelevant since you would be flying on AA. I though that the BAEC search engine now automatically includes partner flights on a search, but if not, make sure you click on the "Include Partners" button if one is offered.