Log into the Poker client and click on 'My Account' and then 'Tickets'. This will show you the details of your current tokens, including which tournament(s) tokens can be used for. Click on 'Use token' to use your token.

You will also often see a 'T' symbol shown next to tournaments in the tournament lobby; these are games your tokens will gain entry to. Just click 'Register Now' on the tournament to use your token.