Your setup looks correct. I tried adding MBUTTON_UP to my Back action and it worked for me.
A few thoughts:
- you are holding the right button while clicking the middle button, yes?
- are you sure your mouse is configured correctly? Most mice these days have a scroll wheel, and "middle button" is only one option for what happens when you press it.
- an easy way to test a gesture is assign the action "On Screen Display" which displays text in big letters across your desktop.