Instructions
Connect to the Internet through your PS3 video game console. You can either connect the Playstation straight to the Internet via an Ethernet cable or a router connection.
Load the "Rock Band 2" game by turning the PS3's power button to "On." If the game is already in the loading tray when you turn on the game, it loads automatically. Otherwise, you must open the loading tray, put the game in and close the tray before the game can load.
Click on "Marketplace" or "Get New Songs" from the options displayed when the Main Menu screen appears.
Scroll through the list of available songs and choose the songs you want to purchase by pressing the "X" button on the remote. You can select multiple songs to purchase at one time. You must have prepaid credits or a credit card in order to purchase songs for the "Rock Band 2" game.
Click "Purchase" to purchase the selected song or songs and begin the download. Once the download finishes, you should see the new songs in your "Rock Band 2" playlist of songs.