Multicast routing on Mac OS X is a bit different from Linux. For some reason a node bound to 127.0.0.1 (lo0) cannot use a multicast route via en0. Usually people run the testsuite on 127.0.0.1 and use a real IP address (e.g. 192.168.1.3) o run apps that needs to talk to other hosts.
To accomplish this, 2 multicast routes have to be added:
# Adds a multicast route for 184.108.40.206-220.127.116.11
sudo route add -net 18.104.22.168/5 127.0.0.1
# Adds a multicast route for 22.214.171.124-126.96.36.199
sudo route add -net 188.8.131.52/5 192.168.1.3
When binding to 192.168.1.3, a multicast address (mcast_addr) from then second range has to be used and when binding to 127.0.0.1, an address from the first range should be used.
See  for details.