I’ve had similar problems to those mentioned above with the DIR-655 firmware 1.20. I am using the DIR-655 as a wireless access point, not my internet router. The multicast option is checked in the configuration utility, but the SB apparently isn’t getting the multicast packets as it reports no Firefly.
I had an old DIR-614+ (also a D-link router) sitting on a shelf, so I plugged it into the same switch that the DIR-655 is on. This router does not have a configuration specifically for multicast (that i can find), yet when I change the preferred wireless network in the SB to the DIR-614+ the Firefly server shows up and everything works fine.
Given the exact same network topology for both wireless access points, I have to conclude that there is something wrong with the multicast implementation in the DIR-655. Possibly it is only set up to pass multicast packets coming off the WAN side, not the LAN side.