First, to be sure your machine can receive mDNS advertisements, you will need to make sure that port is open on your firewall. For example, in Fedora, you can go to System->Administration->Firewall, and make sure mDNS, UDP port 5353, is open. (If this tool isn't available, 'yum install system-config-firewall').
Now, to see the libvirtd instances on your local network, bust open virt-manager, and head to File->Add Connection:
Move the second drop down to select one of the 'remote' options. This should activate the browser window and auto-populate any local connections:
From there, you can just select an entry in the list and hit 'Connect'.
FYI, you can customize the string that libvirtd advertises. Edit /etc/libvirt/libvirtd.conf and look for 'the mdns_name' option. Uncomment and put in your own value. Restart libvirtd with 'service libvirtd restart', and the new string should be advertised:
At the very least, your local machine should show up in the list. If you aren't seeing expected results, you can try:
'avahi-browse _libvirt._tcp' If the results differ from what you see in virt-manager, please file a bug. (avahi-browse requires the avahi-tools package on Fedora)
If that produces nothing new, make sure that libvirtd is running on the machines you expect to see, and that mdns_adv is commented out or equal to 1 in /etc/libvirt/libvirtd.conf (should be the default).