My port maps successfully, but it is still closed?
Make sure the OS X firewall is either disabled, or you have allowed Transmission's port. The firewall can cause the port to remain closed, even if it has been successfully mapped by the router.
If you have multiple routers in your home network, you must forward Transmission's port on all of them, otherwise it will continue to show up as closed. Transmission can only automatically port map the router the computer is directly connected to. Any others in between this router and your modem will have to be forwarded manually. For detailed instructions, click here.