I have the same prism.
You have to turn the shutter knob on the RZ to the red circle thing if you want to power up the prism. Once it's set there, the shutter speed is controlled by the prism and you get an indication of the metered speed as well as the speed you've selected. If the RZ is set to anything else the prism isn't powered and it works just like an RZ with a plain prism.
Once you set it to the red circle, you can either choose a shutter speed manually on the prism, or set it to A which is aperture-priority autoexposure. AEL is the same thing but with AE lock as long as you hold down the shutter halfway.
I'm too cheap to buy the double cable, so I just use a regular cable screwed into the lens and trip the shutter button with my finger.