You'll be instructed to connect your Palm Pre to your computer using its USB cable. Go ahead and do so.
You may see your computer installing device driver software to connect to the phone; let this run.
When your Palm Pre is successfully connected to your computer, you'll see three options appear at the bottom of its screen: Media Sync, USB Drive, or Just Charge. Select "USB Drive" and then click "OK" on the Pre's screen.
Go back to your computer and click "Next."