How to stop the 'd' key from minimizing all windows in Ubuntu 10.10 when connected with Nomachine?
By default Ubuntu maps the "show desktop" command to the Super+D key combination. The Super key is also sometimes known as the Windows key. While I've never used Nomachine, my assumption would be that it is having an issue with the Super key as it does not exist on your Mac. I would try changing that key binding on the Ubuntu side.
Go to System > Preferences > Keyboard Shortcuts
Find the entry titled "Hide all normal windows and set focus to the desktop" and try a new keyboard combination.