Players have reported a few ways to solve this problem:
Forwarding all the UDP ports in the range 1024..65535 (a lot I know, Flash will pick one at random from that range).
If you are behind two routers enable UPnP in the first one.
Configure your router to point to your computer as a DMZ host.
Removing your router from the equation entirely and connect directly to the internet.
If you are unable to enact the above fixes, don't worry, there are some hosts that you will always be able to connect to, make sure to try every game available. You will be able to connect to at least one of them.