Let me explain how I do that, and then you will say if you are doing the same.
1. Open ME, settings. I see "middle button click" default configuration for "Mouse button configuration".
2. Move mouse pointer over textbox under "Mouse button configuration" label. Text is changed to "Press any mouse button...blabla" :)
3. I (this is a working example) press down Alt key and with keeping it pressed click with left mouse button onto this textbox.
Message is changed to "Alt + Left mouse button".
But when writing this guide, I noticed, that, after setting "Alt+Left click" I cant change it's value anymore until settings are reopened.
I will see where the issue originates from.
Btw, what keys did you try?
Sincerely yours, Alexander