If you're syncing using iTunes, you'll need to use the database to access your music files in Rockbox. To make sure it's updated, go to Settings>General Settings>Database and either initialize the database if it's not already initialized, or select the option to "Update Now" to update the database. Note that updating the database will require a restart of the player to commit the new database files after it's been updated.
You can also tell Rockbox to automatically update the database by turning "Auto Update" on.