just read the Ground Control manual..
note I don't have one of these myself...
first you have to turn on the expression pedal
so jack the pedal into 'pedal 1' and turn it on
from manual:
Enter Setup mode by pressing both [SETUP MODE] buttons at the same time. Then select this category by pressing [PEDAL]. The display should read:
To enable (turn on) the pedal connected to “PEDAL 1”, press [+/YES]. To disable (turn off),
press [-/NO]. Press [SELECT 2] to access “PEDAL 2”.
then you need to setup the MIDI side of pedal 1
from manual:
USING EXPRESSION PEDALS
Before using expression pedals, you need to enable them from the Pedal category of Setup mode.
After you select a preset and enter Edit mode, you can specify what each expression pedal will control by pressing [SELECT 2] until you see the following display:
Pedal
This tells you which pedal you are editing. Press [SELECT 2] again to select pedal #2.
Current Value
This shows you what position is being read from the expression pedal. It’s really just a diagnostic tool so that you know if the pedal is working properly. Wiggle the pedal and make sure the numbers change and your device is responding properly. If the numbers are changing, then the Ground Control Pro is definitely sending the specified MIDI messages.
Clarky EDIT: this number represents "where" the pedal is in it's throw from:
min [heel down = 0]
max [toe down usually = 127]
and if you move the pedal to somewhere around half way you'd expect to watch these numbers changing and end up with number between 50 something and 70 something [the exact mid point = 63]
Message Type
Now you need to choose a MIDI message type. It doesn’t matter which you choose as long as it matches the message type that your device is set to respond to. The choices are:
• No output: “OFF”
• Pitch Bend: “PITCHB”
• Aftertouch: “AFTERT”
• Continuous Controllers 000 through 120: “CTL007”
Clarky EDIT: you want continuous controllers. The numbers here match the controller numbers in the Axe: I/O: CTRL page
so using my presvious example, set this to 16 so your "pedal 1" wiggles show up in the Axe on modifier: Extern 1
To choose the MIDI message type press (or press and hold) [-/NO] or [+/YES]. Some devices let you assign controller messages to whatever parameters you like. Others will have a list of what each controller does. You need to check the manual for the device you are controlling to see how it works.
Tip: Controller #7 (‘CTL007’) is almost always volume.
Clarky EDIT: CC# 7 is actually very specific. It's "master volume". That is the volume of that whole unit [fx unit or synth etc]. So assigning to this is like altering the volume via the output1 / output2 controls on front of the Axe rather than having a vol pedal at the start of an fx chain.
If you want vol pedal control within the Axe, add 'pedal 2' to the Ground Control, turn it on, assign it to CC 17, which becomes Extern 2 within the Axe. Then assign the vol parm [within the vol block] to modifer Extern 2]
Target Device
If you have more than one MIDI device, you must specify which one the Expression Pedal should control. Press [CURSOR 4] to get to the following display:
Press [-/NO] and [+/YES] to select the correct device. Only those devices that you have specified in the Device Name/Channel category of Setup mode will appear.
Clarky Edit: this only applies if you are controlling more than one FX unit [or MIDI 'thing'] from the Ground Control Pedal. If the Axe is the only thing being controlled I'd assume this would be device 1 and that most device specific things within the Ground Control would be defaulted to device 1.
That said... if some things still don't work, check that device 1 is on MIDI channel 1, and all the preset buttons, instant access switches and continuous controllers you use are assigned to device 1
again.. I don't actually have one of these units.... some I'm just trying to help you guys through the manual...