The manual is online, not sure why you are struggling to download it?
In terms of the circuit, they have shown one with a simple Op Amp. I agree, its not that difficult.
The RS485 way is fairly easy. I had a plugin before on Mach3 which worked fairly well where I used USB / RS485 - 2 wires going through to the VFD.
Ill try the RS485 adaptor tonight assuming its delivered today.