How can I assign the key Super+M» to minimize all windows in Ubuntu 12.04?

A quick workaround would be to remove the Unity Music Lens.

Open a terminal (hit Ctrl+Alt+t) and type:

sudo apt-get remove unity-lens-music

You will be asked for your password. Type it in and hit Enter.

Then log out and log in again.

Now you can assing Super+M to minimize all windows through System Tools ▸ Keyboard ▸ Shortcuts ▸ Navigation ▸ Hide all windows

You need to modify the *.lens files to change the hotkey or disable the lenses. More http://ubuntu-answers.blogspot.com/2012/05/change-dash-lens-hotkey.html

Quick solution is:

Use the shortcut: Cntrl + Super + D

Super is the same key with Windows logo generally or the function key for the laptops.

Hope this helps.