1st, read the doc here :
http://managames.com/tennis/doc/Tennis_ ... ml#network
After, you can click on "Connection test", after you have created a game, to know if your game is joinable or not.
If not, it'll mean you'll have to open your UDP port 4321 (see link above for further explanation).
If there's something you don't understand, or you cannot make it work on your PC, then ask about it...