Slide your finger left on the display to open the main menu.
Tap YouTube.

You can now use the application.
YouTube isn't part of the original application package and therefore only a general description is given here.
You have a number of options, such as:
Search for video clips
Search for video clips in a category
If you have a YouTube account and you log on, you can also:
Share video clips
Create playlists
Rate video clips

Tap the home icon to return to the home screen.