Buy a microSDHC card (you can get a 4GB for under $20 these days).
Set-up Mass Storage Mode on your Pearl, connect to your PC, where you'll see the Pearl's card mounted as a drive (probably E or F). Just drag/drop your MP3 files onto the card. Voila!
Thanks, but is there a way to just transfer via a USB cable or something like that?
Yes, that's what the first response was referring to. Without a Media Card your Pearl will overload it's memory quite quickly, which will result in lost messages, logs or worse. Once you've got a SD card installed and set up, you hook up to the computer via a USB cable, then simply drag and drop (or copy/cut and paste) to your Pearl. It will behave just like a memory stick. Here's how to set up your card, the install I believe shows under your SIM card, but you've got a convenient side port. Lecture 6: How to Install a Media Card into your BlackBerry | CrackBerry.com
Here's what size SD you can put in your Pearl: http://forums.crackberry.com/f3/tip-...ckberry-91909/