When you go to Cinnamon Settings > Keyboard > Keyboard Shortcuts, and in the window on the left under Categories you select Cinnamon,
then in the right window under Keyboard Shortcuts there is an option for "Menu button". I assume you are not seeing this option?
If not, have you tried connecting an external USB keyboard to see if the option now appears? This keyboard should function along with the
built in one. You may have to restart your system.