After searching for a solution I came across this article in de rokulabs forum
http://forums.rokulabs.com/viewtopic.php?t=10805
And I having the same experience: after enabling wifi on my iMac the firefly server pops up on the roku. After rebooting the roku I can sill select the firefly server.
I disabled wifi on the iMac again and firefly disappeared on the roku. ❓
I am confused: multicasting is working but only when I enable wifi on my mac ?
Thanks Wouter.