I cannot get Transmission's port to open!

If your port is still not open, even after you have enabled automatic port forwarding, here are some tips you can use which may get it working.

UPnP

For UPnP compatible routers, make sure:

UPnP is enabled. Consult your router's documentation for instructions. If your router doesn't support UPnP, you will have to forward manually.
DMZ mode is disabled.
The port has not already been forwarded manually.

If you are still having problems, open the Message Log (in the Window menu) and post the debug output on the support forums. Make sure you pause your torrents first; then clear the log, and toggle "Automatically forward port". Post this output.

Airport

If you have an Apple Airport, make sure NAT-PMP is enabled.

To do this on Tiger/'Airport Admin Utility':

Open Airport Admin Utility
Select your base station and click "Configure".
In the "AirPort" tab, click "Base Station Options...".
In the "Ethernet Port Security" tab, check the "Enable NAT Port Mapping Protocol" checkbox.
Click "OK".
Click "Update".
NB: NAT-PMP requires MacOS X 10.4 Tiger, as well as up to date Airport software and firmware.

To do this on Leopard and 'Airport Utility':

Open Airport Utility
Select your base station, and then choose Manual Setup from the Base Station menu. Enter the base station password if necessary.
Click Internet in the toolbar, and then check the "Enable NAT Port Mapping Protocol" checkbox.
Click "Update".

Double NAT

Another possible reason your port remains closed could be because your router is not the only device on the network which needs to be configured.

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.