Here you go, have a look at the attached. This shows the procedure (last to first) of obtaining a 'masking' channel for the item I want to isolate.
Create an object. Create a render tag and drop it onto the object. Apply the Object Buffer to it. Go to render settings and enable multi pass and object buffer. Render it out and now I have an alpha channel in my document which I can use to 'cut out' the object.
Hope that makes sense!
I can give you this simple c4d file if you like?