Tima84,
Most of these cadet assessments are looking for Pilapt skills. If you use a joystick with FSX it'll help you in that area, especially for the 2D and 3D skills. Another useful resource you might consider is the Skytest or PilotAptitudeTest.com. These will get you comfortable practicing the more basic pilapt tests that most cadet assessments use.
For the group exercise, don't be overly dominant. Make sensible comments and observations during the exercise. You'll often find some people jump straight in and want to show their leadership skills. They often lose marks as they're speaking before considering the options properly or listening to others. Keep your mouth shut for a few minutes and let everyone speak. Also, keep a close eye on the time and tell the group you'll keep an eye on time. This shows you're observing the group exercise time requirements and will score you a few points.
In the last two minutes, call the group together and say "Ok, let's just summarise here and get a final answer Mr. X, you made a good point about this topic, Ms. Y, you also made a good point about that topic. Can we agree these are good ideas and go with them?" Even if they disagree with what you say, the team observing will notice you're pulling them together to get to an answer, which is what they're looking for.
Whatever you do, don't argue during these exercises. Keep calm.
Hopefully this is of some help.