I try to assign the down and up switch to trim since there already are text on the stick saying NOSE DOWN and NOSE UP. But in FSX the hat switch seem only to be identified as one switch called HAT SWITCH. How do I assign the eight different positions ?

I don't think it's possible within FSX, but you could assign them using FSUIPC or the Thrustmaster profiling software provided with the Warthog.

 

The commands to look for in FSUIPC are ELEV TRIM UP and ELEV TRIM DN.