If you are new to Mancala, click on "Connection->Register". You are asked for an username and password. After clicking OK the program tries to register you at the server. Probably the name you have chosen is already reserved. Then try anopther one. When you are successfully registered you have to connect to the server, see above.
When you are connected, you see all players, that are online at the moment, on the right side. You can choose one of them and send them a chat message by entering the message in the text field at the bottom. If you want to play a game with somebody, select his name at the list, then click on "Mancala->Invite". The other user is asked if he or she wants to play with you. You get a message, if he or she wants to play with you. If the other user is playing at the moment, you get the message, that he or she don't want to play with you. Never mind, maybe later. Or choose another user.
If somebody wants to play with you, you are asked, if you want to play with him or her. If you answer "ok", the game starts immendiatly.
The game is played by clicking at one of the buttons ("holes") at the lower - your - row of buttons. The game ends, if one row is completely empty. If you want to stop a game, that you are playing (altough this isn't nice at all for the other player), click on "Mancala->Stop Game".
I think that's all... Have fun!