I have the CH Flight Sim Yoke and Rudder pedals also MS Sidewinder Precision Projoystick.I tried to assign the throttle on my Sidewinder to elevator trim axis. In settings it is marked as trim axis. But it won't work.But I could assign a axis on my CH Yoke as trim. My sidewinder is recognised as well as my assignment of it's throttle as trim axis.So it should work?!But FSUIPC does not recognise the MS stick throttle as trim.

Have you adjusted the sensitivity of that axis in the Joystick settings of FS? Some of them default down to 0 when you add an assignment which prevents it from working. You need to move the slider up.