Can you view the calibre server page from Safari? Try http://your.calibre.ip.address:8080/opds
(change "your.calibre.ip.address" to the ip address of the machine where you're running calibre, and "8080" to the port you chose -- 8080 should be the default, but if you changed it in calibre change it here. You also need to use /browse rather than /opds, as safari will probably attempt to redirect you to the reader.mac.com RSS feed reader, which will fail because an external internet site won't be able to hit a server inside your local network). If that doesn't work, the firewall on the calibre machine is probably blocking in-bound access.
If that does work, then something else is blocking the UPnP messages that Stanza uses to find calibre servers. You can either try to track that down (could be a firewall issue, could be network segmenting, could be something else), or you can manually add the calibre server to Stanza by putting in the http://.../opds
url I mentioned above using the "Add Book Source" option (on the Get Books page, Shared tab, tap the "+" in the top right corner).