Instructions
Log in to your account on the PlayStation Network by choosing "Change User" from the XrossMediaBar (XMB). You will be prompted to enter your login name and password.
While signed in to your account, access the PlayStation Store from either the Games or PlayStation Network column of the XMB.
Navigate to the top menu and select the icon with an arrow pointing down. This icon accesses your Download List, which is a complete history of all items you have downloaded from the PlayStation Store. All items in this list will have the PlayStation Store's "blue bag" icon colored red, indicating that they have been purchased.
Scroll down until you find the DLC you wish to use.
Select the DLC you want and choose to download the item. You will not be charged for additional downloads once the DLC has been purchased.