Instructions
Go to the official PSP Revolution website (see link under Resources; while you're there, make sure that you have the latest version of PSP Revolution installed on your PSP).
Go to the "Downloads" option, click on the "PSP Revolution" option, then "Songs."
Choose from the categories of songs, including "Plaguemix Antique," "Plaguemix One," "Plaguemix Two" and "Plaguemix Three."
Click on the song you want to download from the list. Details about the file will then appear on a new page. Click on the "Download" button.
Start your PSP in USB mode and connect to your computer using the USB connector.
Locate the song file and transfer it to your PSP. You can now play that specific song in the PSP Revolution game.