I updated to 11 for free not sure if that is possible on older macs.
Yoda, GarageBand version 10.0 is the newest GarageBand. The GarageBand '11 you are running is version 6.0.5, so you might want to upgrade. The upgrade is free, but requires MacOS X 10.9 Mavericks. And if you upgrade, make sure not to overwrite your current GarageBand '11, because of some of the missing features.
You can use SoundCloud (If you have one)
I use soundcloud to show my music/loops.
to convert Garageband songs to MP3, use Soundcloud. Upload the song to SoundCloud then go edit your song settings. Make sure that you have Downloads Enabled! Next, download your song. Type: "m4a to mp3" in the search bar. export your garageband song file to the website and follow the instructions. Lastly, download the converted file.
I hope this works if you use soundcloud
As of March 20, 2014, Apple released Garageband 10.0.2.
The ability to export your files to MP3 format is BACK!! I've been using it for the past week and it works fine.
The only thing that I don't like about it is that it appends the date and time of the export to the end of the audio's file name just before the ".mp3" extension. As far as I know, there's no way to turn this feature off. Every time I export, I have to first edit the file name BEFORE I click on the "Export" button.