To do manual volume swells with an Expression Pedal as a Volume Pedal on the M13:
Plug your Expression Pedal into either Expression Pedal Jack 1 or Expression Pedal Jack 2* (make sure your pedal is compatible with LINE6 devices - they are not the same as those compatible with BOSS devices).
Press and hold the switch that says "Hold for Setup" (Looper Controls).
Locate the parameters in the first LCD screen (FX Unit 1). Turn on the Expression Pedal by assigning it's Expression Pedal Jack # to one of the four FX Units*.
On the FX Unit # you assigned your Expression Pedal to, press one of the three footswitches, then on the selector knob Press For Type (Yellow Type of effects [Distortion]), then turn to Select Model (Volume effect).
The display LCDs should move as you treadle the Volume Up and Down with your foot..
Have fun.
*M9 may not have two Expression Pedal Jacks and may have 3 FX Units not 4.
**To simplify the process, you can choose Jack, Unit, and Footswitch "1".