You're probably running the old packaged version of plugui (which is what that web interface is called), and there's a high probability you're using Internet Explorer to access it which usually doesn't work
The share stuff in plugui is unrelated to samba/dlna, it's a simple way to share single files with people over the internet, and some of the pieces that make it all work smoothly are currently broken (has to do with the way individual plugs are identified for the pseudo-DNS service at http://plugfinder.appspot.com
If you're just interested in setting up minidlna and samba, follow these guides: http://archlinuxarm.org/support/guides/applications
If you have trouble with those post back here or stop in the IRC room and i'll help you.
PlugUI is still being worked on and overhauled but at the moment you shouldn't rely on it especially if you use internet explorer