First, be sure that the pedal is connected to the EXP2 port (says EXP100 only below that jack).

1. Press Menu/Exit
2. Select PAGE A (with the Page button)
3. Select the Control [4] button
4. Use the up and down arrows until you see "Express."
5. At this Expression page set SOURCE to EXP-100
6. Move the foot pedal up and down and watch the MONITOR display, it should be moving between 0 and 127.
If you see the Monitor numbers moving as you move the medal, it's a good sign that the pedal is working.

If the pedal does not control volume, reset the XK3 keyboard.
You can do this but pressing Menu/Edit again, and selecting Page E. Touch the [1] Default button and use the Parameter button to choose ALL. Press the OK soft key and the keyboard will be reset. (Not that this may remove any user programs or setups you've customized).