If it's the "Super" top-fire, you have to pull really hard on the disk piece at the bottom that contacts the fire microswitch.
Add a small amount of twist while you are doing it to get the best results.
It is connected to a plastic shaft inside, so don't get TOO violent with it though.
After you've had it apart a few times, it's alot easier.
Look at the exploded view
HERE to see how it all goes together.