Assigning Hotkeys and Mouse Buttons to ToolbarsKeyCreator \ Tools \ Customize \ Commands \ Assigning-Hotkeys-and-Mouse-Buttons-to-Toolbars
To assign a specific function to a mouse button, find the Category for that function in the Tools>Customize>Toolbars, then click on that function when its program icon appears in the Buttons list. Then, click on a mouse button to complete the assignment.
For instance, to assign the Undo function to the Extended 1 mouse button, first scroll the Categories list for the Edit category, then select the Undo function icon. Notice that the function description appears under Description. Finally, select the Extended 1 mouse button followed by Assign Key.
NOTE: You can also combine Shift or Ctrl keystrokes with button assignments to create more combinations. For example, you could assign the middle mouse button to the Conversation Bar Backup function, and the Ctrl key + middle mouse button combination to the Undo function.