OP did you ever figure this out? Because I have a pedal version of the hum extractor and decimator and I am trying to get it to work with my AXE FX 3 and I am having a terrible time getting it to work.
Hey what's up! Yes, I figured it out and it's actually quite simple. Two things already in this thread that are usable are my initial wiring diagram on white paper (use only ISPL wiring) and Moke's last screenshot.
Remember that the hum extractor and decimator are used in series not parallel. You'll need to use AF3 physical INs and OUTs of just 3L and 4L since you only have one ISP unit. AMP and CAB inputs will also need to be Left last well. No need to actually pan them if you're not actually doing dual guitar sources (stereo) like in my setup.
So first you'll need two humbuster cables since you're coming OUT of 3L and 4L. Then two standard TS cables to go IN to 3L and 4L. I made my own cables for this using switch craft & Neutrik connectors and Canare quad cables, Canare has braided shielding which to my understanding is better than any other. Neutrik connectors seem more friendly to work with.
Physical wiring
1. TRS humbuster cable AF OUT 3L (TRS) into ISP Channel 1 IN (TS).
2. Standard TS ISP Channel 1 OUT (TS) into AF IN 3L (TS).
3. TRS humbuster AF OUT 4L (TRS) into ISP Channel 2 IN (TS).
4 Standard TS ISP Channel 2 OUT (TS) into AF IN 4L (TS).
5. Set the physical knobs on front panel of 3 and 4 on AF to max. (This works in my case, should be the same on yours)
AF Global Settings
1. Setup>Global>Input Levels - A/D Sensitivity 1 and 2 you'll figure out for your setup. Input 3 @ 100% and Input 4 @ 60%, this I'm guessing will be the same for you. (You can do this in Axe Edit accessed by clicking the little grey arrow to the left of a selected block in the edit section.)
2. In Setup>Global>Audio - Output 3 Boost 18dB, Output 4 Boost 18dB
3. Setup>Global>Audio - INPUT CONFIG for 3 and 4 I have MODE set to stereo but you may need them at LEFT
AMP & CAB Blocks
1. AMP "Input Select" LEFT
2. CAB "Input Mode" LEFT
I/O Blocks, AMP and CAB
Series block path as follows - IN1>OUT3, then no connection because it's wired in the back, then IN3>AMP>OUT4, then no connection because it's wired in the back, then IN4>CAB>(whatever time effects you want)>OUT1
PITCH Block
One additional thing about the ISP is that it doesn't work well if you put PITCH Virtual Capo between IN3 block and AMP. You'll need to put it between IN1 block and OUT3 Block. I put my wah there too.
See attached screenshots.
I hope this helps and feel free to ask anymore questions about it!