Ok, here it is. My version of Expose only shows 4 things in the "Screen Corners" section. (Makes sense, unless your screen has five or six corners.) Those are: Start or Disable Screen Saver (that's 2), Sleep, and Spaces. Now, before there were Spaces, there was void - oh, wrong text... There was something else.
Then in the Expose section (strange that a section has the same name as the toggle button for Expose/Spaces) is where you set F keys to show All Windows, Application Windows, or the Desktop ("naked"). Then there is Dashboard below that. Next to all of these is another pull down with choices for a Secondary Mouse Button, or Mouse Button 3 4 or 5, and those are blank (instead of Some Windows, Some Applications, Something Else...). I don't know if I can define something instead of the 'dash' it shows there now. When I pick the pull down, it reveals a check mark next to the dash, and the mouse button options.