I'm assuming you're talking about BTSX, but the procedure is similar for keyID/DNS and probably for bitMusic/NOTES.
In any case, just open up the client software, log in and create an account (doesn't need to be a registered name). Next, goto the "keys" tab for the account and choose the "import keys from wallet" sub-tab. There's a file selection menu where you can select the wallet from which you donated to AGS (either BTC or PTS). Put in the password for the wallet, and the client will verify your private key and deposit the corresponding amount into that account.