Instructions
Exit "Castle Crashers" if you're currently playing it. Go the PS3 system menu.
Select the "PlayStation Network" and then "PlayStation Store."
Push your analog stick right until you highlight "Search." Click the "Analog" button. A virtual keyboard appears. Type in "Castle Crashers."
Choose which of the "Castle Crashers" DLC you want to install.
Highlight the "Download" button and then push your analog stick to begin downloading the DLC. When it's done downloading, it will automatically install.