The user guide (manual) is included in the RADMC-3D package. If you are interested to have a look at the manual before downloading the whole package, you can get it here:
But beware that this may not be the latest version (for that: please download the main package).
The Python front-end was developed by Attila Juhasz. The manual for that package is separate:
Attila Juhasz also created a tool to map SPH models onto a RADMC-3D grid: