Setting up expression pedals on a Mastermind to control an Axe-Fx III requires a few steps. First, make sure that you've set up the Mastermind to control the Axe-Fx as described in the manual, or in one of our videos.
If you haven't done so already, go into setup on the Mastermind, select Edit Devices, then select Axe-Fx III, and finally select Set Default Pedals. This sets up the expression pedals to send the correct MIDI CC messages.
Then, in the Axe-Fx III setup, go to MIDI/Remote > External. Set Ext Control 1 to CC11, Ext Control 2 to CC12, Ext Control 3 to CC13 and Ext Control 4 to CC14.
Now, the remainder of the setup is done in the Axe-Fx itself. You can assign the external controllers to control different parameters in each preset. In the Layout screen, go to the desired effect and press Edit. Select the desired parameter "knob" on screen. Any parameter that has a yellow dot at the bottom of the knob can be controlled. Press Enter, then set Source to Ext Control 1, 2, 3 or 4. (for expression pedal 1, 2, 3 or 4 respectively). You can set up multiple parameters to be controlled by each pedal.
For more details, please refer to the Modifier Tutorial: Wah Pedal section of the Axe-Fx III manual.