If it doesn’t show up in your shared libraries, then it’s a multicast issue.
You can run mt-daapd with -d9 and a logfile specified in the config file, but I can’t imagine you’ll see any more information — it’s apparently working, since you can see and use it from the mac.
All you can probably do is run a sniffer on the XP box and the linux box while starting iTunes — see if you can see what’s not going. See if the linux box isn’t getting the mdns queries, or if it’s not responding to the mdns queries, or whatever.