After downloading and installing Renderman to its default location, the simplest solution is to put following lines into your Houdini.env as well the Rmantree as a seperate entry in your System variables and appending the path with the @Rmantree. But! in the path variables of your System use $ instead of @. Forward slashes are valid for all three targets.
RMANTREE="C:/Program Files/Pixar/RenderManProServer-21.2"
Path=$PATH;@RMANTREE/bin
Thats it.
Cheers