Instructions
Turn on your PS3 and navigate to the "PlayStation Network" icon on the home screen.
Press the "X" button to enter the PlayStation Network. Click on the "Search" box on the right side of the screen.
Use the onboard keypad to type "Castle Crashers" into the search box. Press the "X" button to start searching.
Click on the "Castle Crashers" icon when it appears on the screen to start downloading it.
Wait for the download to complete and then click on the icon under "Games" to play the demo.