This has to be done for each preset where you want to have wah assigned to the expression pedal.

1. Press 'Edit'.
2. Turn the main data wheel until the screen shows 'Wah'.
3. Turn the #1 dial under the screen until the Wah is turned On.
4. Turn the #2 dial so the small arrow moves next to the pedal parameter.
5. Press 'Assign'.
6. Rock the expression pedal back and forth.
7. Use the #3 and #4 dials to set the minimum and maximum values.
8. Store your changes (press 'Store' 3 times).

That's it. If the pedal had previously been assigned to volume, then it might not sound right. You'll need to unassign it from Volume.