Adding MP3 Files or Folders from the Hard Disk
You can add MP3 files from the hard disk in any of these ways:
With the Audio tab selected, drag MP3 files or folders from any location on your hard disk to the Audio content area.
With the Audio tab selected, click the Add button. The Open dialog appears, where you can browse your hard disk for MP3 files. Select the files and folders you want to add, then click Open.

See Also:

Overview of Making an MP3 Disc
Adding MP3 Files from a Data Disc or Existing Disc
Adding MP3 Files from iTunes