To use the Rick-o-Sound, you'll need to get a stereo to mono splitter cable (also many times called a Y cable). Make sure it's a cable that goes from a quarter inch, stereo male plug to two, quarter inch mono female plugs. Then just plug in a cable into each female plug and you'll be all set.
Is it worth it? Eh, it's a fun little thing to fool around with, and the cables don't cost much at all. I believe Rickenbacker sells a special adapter or kit or something for all of this, but you really don't need it. You should be able to pick up these cables for a lot cheaper at any radio-shak or decent music/stereo store.