First off, you need to change the FX MULTIMODE CONTROL in the Master menu to PRESET ON MIDI CH.XX. The FX realtime controllers default to MIDI controller K (CC91, or Continuous Controller #91) for FX engine A and MIDI controller L (CC93) for FX engine B on the front panel real time control knobs, so any external controller sending data on CC's 91 and 93 will adjust the FX mix level. You can change which CC you have assigned to MIDI controllers K & L in the Master menu, and which FX send you adjust with a particular controller in the Preset Edit menu.