<p>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.</p>
<p>If this doesn't happen, you can add Transmission to the firewall manually:</p>
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<ol>
<li>Open System Prefs → Security → Firewall. Make sure "Set access for specific services and applications" is selected.</li>
<li>Click the "+" button and select Transmission from your Applications folder. </li>
<li>Make sure the pull down menu is set to "Allow incoming connections".