Do you have film in the back?

I actually just got a RB67 in the mail from KEH yesterday and kind of went through the same thing playing around with it. I didn't have film in.

Make sure the shutter is cocked in the lens and the mirror is cocked in the body when installing the lens. To cock the shutter, turn the pins all the way to the red dots.

IF you have a Pro S - If there is no film in with the back on, remove the darkslide so the triangle appears. By the film advance knob, you will see a little lever. It disables the double exposure interlock. Move it up so a red dot appears. You will now be able to fire the camera and recock the shutter.

hope this helps