I am using the Saitek Pro Yoke and an additional flight control (throttle) unit. I have assigned the lever axis on the yoke to spoilers - throttle 1 - throttle 2. on the additional unit the levers are assigned to throttle 3 - throttle 4 and flaps, via the FSX options - controls menu.

For reverse thrust use a REGISTERED copy of FSUIPC. Select the "Buttons and switches" tab. If you are using only one lever for all 4 throttles move that lever down through the detent and you will see two numbers in the box named "joy". You will see something like joy # 2 20 Button #. Put a check mark in the box "select for FS controls" and select throttle decrease. Then place a check mark in the box named "control to repeat while held.

If you are using a lever for each throttle the procedure is the same except select throttle 1 thru 4 for each lever in the  "select for FS controls" drop down.

That's it. Move the lever(s) tback through the detent for reverse thrust and foward through the detent for normal.

Hope this helps.

Ian