From what you've said so far, it seems like your signal chain and FX loop point (post FX2) setup is correct. Make sure your M13 is set to DSP Bypass. Then connect your guitar, M13 and amp as follows:
Guitar > M13 Left Input
M13 Left FX Send > Amp front input
Amp FX Send > M13 Left FX return
M13 Left Output > Amp FX Return
Make sure the amp's FX loop is set to serial or that its mix control is set to 100% thereby making it a serial FX loop
In the setup screens you simply use the physical encoder knobs (under the LCD screens) that are in the same positions as indicated on the LCD screens to alter functionality
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