The AAC files won't be read in your Mustang, you'll have to convert them over to MP3. If you go into your iTunes preferences and change the Import settings (under Advanced>Importing) you can change the MP3 settings to the highest settings so you don't lose all that much quality. Then, just select the files that you want to convert to MP3 and under the Advanced menu in the normal iTunes window, just select "Convert selections to mp3" and you're set.
Then, make sure in iTunes that the burning preferences are set to MP3 disc.
If you have a Mac, I'd suggest getting a copy of Toast Titanium for the actual burning, though. Lots more features and you can make sure when you burn a normal audio CD that you get the full CD TEXT feature so you can read what track is playing on the display.
I'm a Mac/iTunes geek, so if you have any questions, let me know, I'll help you out all I can.
jason