Hi Denis,
My sim is on another computer so I will again try this from memory.
In FSUIPC Buttons and Switches tab:
Click rescan then press the button you want to assign. You should see it identified.
On the right side of the box, place a check in the FS and PM boxes.
In the drop down box, scroll down until you see the PM TCAS functions. Click on the one you want.
Close the Buttons and Switches tab and see if you can control TCAS.
Not all PM functions are in the drop down box. Pete Dowson added a drop down function called "Assign By Pram" or assign by parameter. There is a list of PM parameters on the PM documentation page.
I'll look at this again when I can get a minute to turn on the sim. I hope this explanation can point you in the right direction.