Ah. The config page is currently broken (and top of the “to fix” list).
The directory “/shared/hdd/data/public” corresponds to the share called “disk 1”.
If you make a directory directly under “disk 1” called “mp3”, *that’s* the directory it wants to share out of. So make that directory, copy some mp3 files in it, and go to the web admin. Hit the link on the status page to scan the directory, and then connect with iTunes or soundbridge, and you should be good.
Yell if this doesn’t get you anywhere.