If your router supports NAT-PMP, UPnP, or you have Apple AirPort, Transmission can map its port automatically. Most routers manufactured since 2001 have this feature.
If you get a red dot and the message 'Port is closed', click here.
If you don't have a compatible router, it is simple to forward Transmission's port manually. For instructions click here.
If you don't use a router, and your modem is directly connected to your computer, you'll need to open Transmission's port in the Mac OS X firewall. For instructions click here.
Note: It is highly recommended you enable the Mac OS X firewall if you are not using a router.
Keep in mind that many DSL modems also function as routers, so port forwarding may still be necessary, even though your computer is directly connected to the modem.