You need to use a Volume block in the preset and assign your pedal to that.
How do I setup Expression Pedals with the MFC-101? | AxeFxTutorials.com
In that video I show how to change XP 1 from Global volume to External Controller 1, but you can just plug I tot XP2 which is already External Control 1 at default settings.
If you don't want to use a Volume block you could attach the volume pedal to the Cab volume, and make sure that the Cab is earlier in the chain than reverb, delay etc