The Extravert takes up to a 32 MB SD card, so my guess would be it gets loaded on to the SD card via USB or an adapter directly in your computer. USB Mode will allow you to see the memory card in the phone as a drive on your computer to copy the music files to and from.
Good luck, and post back if you have further questions.
Is there any way to upload music, or any media files, onto the phone from the computer WITHOUT using an SD card? Or are we pretty much forced to get one if we want to use ringtones outside of what's already preloaded on the phone?