you mention your "electric signal." do you have a piezo signal as well? in other words do you have 2 completely separate rows in a preset?
are you using the mfc or other controller, or is the pedal plugged into the axe fx?
i'm assuming you have your expression pedal into XP1 of the MFC. that is default set to control Out 1 volume. it has nothing to do with a volume block put in a preset.
if you want to use XP1 to control the volume block in a preset (it's how I do it) you have to change a few things.
first on the mfc press edit, then midi (#4), then press page left 2x (#6). This should take you to a screen that says XP 1 #11. Press Parameter Right (#12) once so the cursor goes under #11. Press UP/Incr until it says #16. The reason for #16 is because that's the CC# your Axe-FX is set to for External 1. More on this later.
Now press Edit again on the MFC to save.
Press the I/O button on your Axe and page right to CTRL screen. Look down the list for EXT CTRL 1 and it should say 16. Again, we set the MFC to match this setting, but you can change it to whatever number you want. I'd suggest leaving the Axe the way it is and just changing the MFC numbers until you are fully comfortable with everything.
Go to your preset and press Layout. Go to your Volume block and hit Edit. The first knob on page 1 is Volume and it has a {} under it. That symbol means you can assign a controller to that knob/parameter. So what we're about to do for volume, you can do for anything with the {} under it.
Ok make sure you are on Volume and hit the Enter button. This takes you to a modifier screen. Here you can choose different modifiers to change the volume parameter. The first item, Source, probably says "None." Turn the value knob to change this to different things like LFO 1A, ADSR, Pitch, etc. We want to turn to External 1, because that's where our Mission pedal is: MFC's XP1 which is CC #16 which is Axe FX parameter External 1... Get it?
Now when you move your pedal, that dot on the graph should move up and down as the pedal moves. Press Store, then enter 2x.
One last step. Press I/O again and go to the MIDI page. Scroll down until you see EXT CTRL 1 INIT VAL. It probably says 0%. Turn the value knob so it says 100%. This means that with your pedal/MFC unplugged, External 1 will default to 100%, meaning full volume. Otherwise it would default to 0% if no pedal was present and you won't get any sound.
To add a volume pedal to other presets, just add the volume block, hit Enter when on the Volume parameter and select External 1 and it should work.
I'd also suggest to check out my videos on electric and piezo setups if you haven't already.
Fractal Audio Axe-FX II | Katsu Kuri Media Blog
You have been extremely helpful, and it was your setup I modeled mine after (same guitar and tutorial videos on axe-fx chains).