Having some issues here myself. Here's what I've done so far - just taking any generic patch and assigning XS1 to the boost parameter.
On the MFC MIDI: XS1 #008 Ch01 TypHDW
On the MFC Setup: XS1 Type Toggle
On the Axe-Fx: I/O-CTRL, selected EXT CTRL 12, hit enter to go to learning, hit the switch, it changed the value to 8.
In the preset: Amp block, Advanced tab, Boost - Hit enter
- Set the source to EXTRN 12
- Min Off
- Max On
-AutoEng Off
The MIDI light flashes when I hit the switch, and in the modifier menu graph, I see the line move bottom left to top right, so the switch is engaging and the CC is being sent, but boost isn't turning on.
What am I missing? I'm using a Mission TT-F.
Edit: Reboot of the Axe and it's working, heh. Of note, like RTCook said, the boost parameter does engage on the Axe, but if you're looking at the edit screen it never switches from Off to On when it engages.