Which trim command are you trying to map to your stick?
There are four commands in the trim switch = nose up, nose down, right wing down, left wing down.
There's also the Set Takeoff Trim command, which corresponds to the button on the left panel near the SAS switches.
There's also a Reset Trimmer command, which I'm not entirely familiar with except that it completely dorks my controls if it is inadvertently mapped to my mic-switch up.
I'm not aware of a single "trim button" on the A-10C.