I can't seem to find how to set a mouse button click to a regular attack. How is that done?
If you have "elective mode" on, you can just drag (with the left mouse button) the skill from the bar and drop it somewhere meaningless (e.g. on the main game area). If you don't have a skill in the "Left click" or "Right click" slot, then the melee will appear there instead. There are no options that I know of to put the melee on a number.
To activate "elective mode", you have to go into Options, then go to Gameplay, and then select "Elective Mode."
So, there you have it! :)