Show Desktop key (Alt+Control+d) is not working in Ubuntu Linux!!

Open CompizConfig Setting Manager from Ubuntu dashboard, and navigate to Key bindings  tab of General options, as shown below:

Find Show Desktop setting. It may be disabled or bounded to some keys-combination.

To set it to keys-combination of  your choice: click the Disable button: an Edit Show Desktop windows  will pop-up:

You can, select Grab key combination, and type your keys-combination. you can use default: Control Alt d. CompizConfig Setting Manager will show it as <Primary><Alt>d

Now you can toggle to Desktop by pressing keys-combination Control Alt d