Configuring the Mac OS X Firewall
Upon opening Transmission for the first time, a Mac OS X dialog box should appear asking if you will allow Transmission to receive incoming connections. Click Accept.
If this doesn't happen, you can add Transmission to the firewall manually:
- Open System Prefs → Security → Firewall. Make sure "Set access for specific services and applications" is selected.
- Click the "+" button and select Transmission from your Applications folder.
- Make sure the pull down menu is set to "Allow incoming connections".