It screws into the filter threads on the end of your lens. The 100mm has a 58mm filter thread size (every lens will have this printed either on the ring on the front element or somewhere on the body usually near the front element. Double check, because there's also a 100mm L series lens by Canon, and it has a 62 or 67 mm filter size.
You need the Canon Macrolite Adapter 58C.
