
There is a green button in the toolbar. But it's not a button - it's a indicator of the Port status (like one in the Transmission preferences menu). I think it is a good idea to put it right to the Toolbar. The same stuff already in Papaya (status bar).
It will show you the Port status - if it's online or offline:
Red icon - Port is closed, Green icon - Port is opened, Yellow icon - Port status unknown …