Along the top menu bar in Rhino, select Tools_Options. Within the options window (for Rhino, not rhinocam), select inches. You may want to save a default document in Inch, since that is what you want to run your machine in.
Be advised, changing units will delete all machining operations.
I looked over Rhinocam settings, and there appear to be none for that. It "knows" what you have selected in Rhino.
There is a post processor generator, and within that, there is an option to scale the output. (Motion Tab of the post processor generator has that. You may scale x, y, and z separately, or leave it set to "1" for default.