The user manual is available inside the RADMC-3D package. However, here you can find the manual of RADMC-3D Version 2.0 built on August 29, 2020 with Sphinx:
Read the HTML manual of the RADMC-3D code package
The same RADMC-3D manual is also available in PDF form.
The radmc3dPy package is now integrated into the RADMC-3D distribution, but it is a stand-alone package with its own manual: