How can I assign "middle click" to a button on my mouse? How can I assign "middle click" to a button on my mouse?  

I am not seeing it in Preferences &gt; Mouse, and every google search just returns results about how to set it up with Apple's Magic mouse.

I am not using an Apple mouse. Specifically I am using this logitech mouse.
Install the Logitech Control Centre System Preference Pane

You het a new Preference Pane "Logitech Control Center"
You will get show attached Logitech Mice etc. Choose the one you want and then you will get a screen to assign buttons to actions