Pretty sure it fits directly to the front of the 100L. I know it does to the older 100 macro and MPE65. I needed to get an adapter for my Sigma 150 macro. But it and the Canon macro lenses are designed to go together.
Be warned, it still gives a bit harsh light. Good idea to diffuse it.
You might be interested in these:
http://nocroppingzone.blogspot.com/2...e-mt-24ex.html
http://nocroppingzone.blogspot.com/2...f-mt-24ex.html
http://nocroppingzone.blogspot.com/2...er-how-to.html
Personally I found those diffusers all a bit too complicated, and just bought 2 Lumiquest UltraBounces and stuck some velcro on the flash heads. Works fine for me.