Adding Audio Files from the Hard Disk
You can add audio files to your audio CD in any of these ways:
With the Audio tab selected, drag audio files 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 audio files. Select the files and folders you want to add, then click Open.

See Also:

Overview of Making an Audio CD
Adding Audio Files from an Audio CD
Adding Audio Files from iTunes