Hello! Are you confused by the names given to the mouse buttons in Compiz's settings manager? If so, this post is for you!
I have not yet tested Compiz with a two-button mouse, but X [size=-2](the graphical server, which you are definitely running if you use Compiz)[/size] usually sets the mouse to be an "emulated three-button mouse". [size=-2](Even real three-button mice seem to be set up this way.)[/size] This means that, if you simultaneously press Button1 and Button3, it will be interpreted as Button2.
On a standard three-button mouse, the buttons are as follows:
Button1 - Left click
Button2 - Middle click
Button3 - Right click
If the middle button is a scroll wheel, the two scroll motions also count as "buttons":
Button4 - Scroll up
Button5 - Scroll down
And, if your mouse also has a horizontal scroll wheel, or you are using a touchpad with a horizontal scroll area,
Button6 - Scroll left
Button7 - Scroll right
If you don't have any sort of input device that offers horizontal scrolling, though, Button6 and Button7 will either A) not exist, or B) do something completely different.
The above should suffice for most users. If you are using a more exotic mouse model with extra buttons, this post may be of limited usefulness to you.
I sense a soul in search of answers!