Manfrotto (Bogen?) makes quick release plates in at least three or four different formats. The most common ones are hexagonal and rectangular. You have to get a tripod head that is specific to that format. There are other quick release type plates too, AFAIK there are Arca types etc. If you post a picture, it will be easier to see what is going on with your setup.

With an RB, you might want to invest in a good tripod and head, as the camera with a lens attached and bellows extended is going to be heavy and wobbly on a flimsy rig. It is the one piece of essential equipment that far too few photographers properly invest in, and it has a greater impact on image quality than going from a mediocre to a great lens.