Are you using POP or IMAP? Some of the following assumes POP:
When you set it up, in the Account Information pane, the E-mail address should be your full address
xxxx@gmail.com, and the User Name should be only the prefix part -- xxxx (without @gmail.com). Is this what you did? In the advanced tab, do you have SSL turned on, port 995, password authentication? (different if IMAP)?