Hi, maybe you've already solved it, if not:
1) Account validation: it's a known issue the devs are trying to solve; afaik, the only solution is to create another account with a different email address.
2) (This is how things work for me in 10.6) To delete software, you need to run osx as admin: just drag the Planeshift folder to the trash (it may ask to enter your password). There could be another Planeshift folder in <user>/Library/Application support, delete it and you're done.
If this doesn't work, make sure you've placed Planeshift in your Applications folder and that you have admin access for it (right-click on the Planeshift folder, select 'get info' and in the new window set the permissions to 'read and write'). At this point you should be able to trash the PS folder without restrictions or, if you decide to make a second attempt, to play the game.
Hope it helps.