Double click on the punkbuster application. A small box should appear. On the top there should be 4 menu buttons: Add a Game, Remove Game, Check for Updates, Quit
if you dont have Battlefield 2 already selected in the box below, go to Add a Game and a smaller box appears. Go to the Game window and scroll down to select Battlefield 2. Automatically the browse window should be filled with the propoer Game Path info. Press OK.
Now your original window should have BAttlefield 2 listed inside. select it and press Check for Updates. The rest should be cake. Good luck.