Yep Basically... just hold the stick gently where you feel you are at level flight and press A once. Let go and you should stay level...that is untill your airspeed changes.

So its not really useable when chasing an Enemy plane?
Edit:
Doesent feel like its working or is it just me?

You can only set it properly in level flight...you see, in the PC version you can setup trim on a slider or a hat switch that you can hold up and down to adjust. It's really hard to do with the 360 setup of holding the stick back or forward and hitting a button to set.

Alright, so it aint a really important feature, more for cruising?

Well it can be somewhat important in BOP sim mode.. think of it like this.. If your plane is steady at a certain speed becuase you are trimmed properly your job at aiming becomes much easier. You are not having to hold back or forward on the stick fighting the plane. Your aiming becomes more precise, especially if the target is a good distance away and not evasive manuevering.

I can tell you from the PC version it is very important when you are making a head on pass at the enemy....once you are lined up from a distance you did not need to make large corrections with the stick. Makes it easier to just point and shoot.

alright thanks, got another question.
Im flying in the mustang, when i push my control stick to the lower left corner, it doesent roll to the left but it starts to spin out. When i do this to the right it does rolls to the right, and so making some sort of barrel roll.
Why could this be happening?

It's because BOP models the gyroscopic effect of torque form the engine. It's why its easier to turn one way versus the other. It's also why the plane will high speed stall easier to the left because it rolls/banks into the turn faster. Try decreaseing the throttle some before the actual turn

Thanks for all your replys! It sure helps quite a bit since the instructions aren't there! Thanks!