first go do your preferences > advanced > importing > import using (change this to mp3 encoder) > settings (apply the settings you want you want)

now, i'm not sure if m4a means that the file has drm on it. but, if it does, you'll need to strip that. the easiest way is to simply burn a disc and then re-rip it: the tracks will no longer have drm attached. and, if you've already changed your preferences, the files will now be mp3s.

if the files have no drm and you simply want to convert them, after you've changed your prefs, highlight the tracks you'd like to convert and, in the menu bar, find advanced > convert selection to mp3

good luck

Ok it sounds like your trying to burn a CD not convert to mp3. Check to make sure you're burning audio CD's and not data or mp3 CD's. Under the Edit Menu select Preferences.

Click the Advanced tab and then Burning sub tab and check "Audio CD" then click OK

When you burn a CD, it will be in redbook format (the standard Audio CD format) and will play on all Stereos that have a CD player.

Hope this helps!