I started using ProTools Free (PTF) a few weeks ago and this is one problem I had to learn how to solve. PTF won't import mp3 files, but it will import wav, aiff, and SoundDesigner II files.

Use Quicktime to convert your mp3 to aiff or wav (I have QT Pro, and I don't know if the free version does these conversions). Run QT, go to File >> Open Movie in New Player, and open the mp3. Once opened, click File >> Export. A "Save As" type dialog box will appear. There'll be a drop down list titled "Export:". Click this and select the file type you need (wav for Windows, aiff for Mac).

In this dialog box is an "Options..." button. Click it and make sure that the "Stereo" radio button is selected, and that the rate is 44.1k (which is audio cd quality). Also verify that "Size" is set to 16 bit. Then give it a name and save it where you need it.

Now run PTF. Go to File >> Convert and Import Audio... . Make sure the bit depth and sample rate match those of your session. Use the dialog box choices to finish the job.

After you're done, you'll have two more audio tracks in your session, and you'll have to name 'em and pan them full left and right for stereo.

The instructions for the ProTools part are in the "Recording" chapter of the ProTools Reference Manual pdf. If you're just learning PTF, reading the "Recording" and "Mixing" chapters are pretty much essential.

Good Luck & hope this helped...