I'm still a dummy with this too! Think of this as foot controllers for dummies, by dummies.
Basically, normal/standard "straight" wiring of a TRS (Tip/Ring/Sleeve - the three distinct sections on the 1/4" plug) stereo cable from one end to the other is:
Tip->Tip
Ring->Ring
Sleeve->Sleeve
For many expression pedals going into the axe (and some other devices), the cable needs to be wired:
Tip->Ring
Ring->Tip
Sleeve->Sleeve
The FV500 will work with "straight" wiring into the axe (unlike many other pedals), but into a MidiMate requires the crossover (?) wiring. Many pedals work the other way around.
As for the H/L thing - I use an H. I believe this doesn't matter for the purposes of expression pedal use, but the H pedal you could also use as a volume pedal for guitar (which is
high impedance) in other situations when deprived of an axe-fx, so is the most flexible option in my opinion.
Using it for expression or volume depends on how you set it up in each patch. I basically have mine assigned to a volume block in all patches, except for wah patches where I have no volume block and it's assigned to the wah instead.
cc