Hmmm....
Well, I only have one real GMail account, but I created two more dummy accounts in PWM with the gmail URL's, and it works fine for me - I get prompted for which account to use, then when I select it, PWM populates my username, then prompts me for my Master Password - then populates that, and I can log in fine...
You said 'when I click on one...' - a dumb question, but - are you dbl-clicking on it? Or clicking on it then clicking 'OK'? You have to do one or the other, or it *will* just sit there, waiting for confirmation from you. After you click 'OK' (or dbl-click on the correct account), your username should get populated, then you should get prompted for your Master Password, after which your password will get populated.