By default Ubuntu 12.04 doesn't have enabled options to show desktop except using shortcut keys Ctrl + Super + D. The "Super" button is actually the Windows button.

However, there are several ways to enable show desktop in Ubuntu 12.04 and we're going to mention the most used.

Show Desktop in Ubuntu 12.04 via CompizConfig Settings Manager (the first way)

1) Install and run CompizConfig Settings Manager

2) Click on Desktop category on left sidebar

3) Click on Ubuntu Unity Plugin

4) Select the Experimental tab

5) Mark option Show "Desktop Icon" in the launcher

The desktop icon will appear in the Unity launcher and you can click on it in order to show desktop

Show Desktop in Ubuntu 12.04 via CompizConfig Settings Manager (the second way - hot corner)

1) Install and run CompizConfig Settings Manager

2) Click on General category on left sidebar

3) Go to the Key bindings tab

4) Find option Show Desktop and click on the button on the right

5) New window will appear where you can select where show desktop will be activated

6) Move your cursor to the corner you have set to show desktop

Using MyUnity to show desktop in Ubuntu 12.04

1) Install MyUnity

2) Run MyUnity

3) Navigate to the Desktop tab

4) Select Show desktop icon.

Show desktop in Ubuntu 12.04 with Ubuntu Tweak

1) Install Ubuntu Tweak

sudo add-apt-repository ppa:tualatrix/ppa

sudo apt-get update

sudo apt-get install ubuntu-tweak

2) Start Ubuntu Tweak

3) Select the Tweaks tab

4) Click on the Workspace

5) Here you can bind four action to four corners of your screen. Click on the drop down menu of any four of them and select show desktop.

6) Move your cursor to the corner to show desktop