LOL, yeah it doesn't really make much sense when you read it. It's easier to understand if you just try changing it through the interface. Here's my brief "idiots guide", I hope it helps!:
1) Press the Menu button
2) On the lcd screen, touch the 3rd icon across the top of the Menu tabs (a small camera icon with bars underneath - the menu colour is orange)
3) Touch the 3rd option in the list, C.Fn III: Others
4) Touch the screen anywhere (it should be on Custom Controls screen/page 1)
5) On the Function Assignment screen, touch the customisable feature you want to assign a function to (the camera diagram on the left will highlight the control you are changing)
6) Touch the same control again to amend the settings for it (in this example we are changing the M-Fn button). Touch each of the icons for a description of what they do.
7) With the function you want to assign to the control (or button) highlighted, touch OK to save it.