You DO NOT have to do the trick that is displayed on the bottom. You can do any of the tricks you've unlocked. It helps alot to pick a trick to do before you hit the ramp so you are ready to the do the motion when the time comes. If you suddenly decide to do one mid air or try to do the one displayed you'll go too fast or just do it entirely wrong.
Make sure you can do it consistently in the ubertricks menu tester thingy first too. When you're in the air pretend you're doing it for the tester, calmly, slowly, and accurately. Once you get a few ubertricks, you'll get the hang of it. The first few are the hardest.