Can I play a game alone?
Yes. There are AI players, against whom you can play. By default, one AI player is created as your opponent. Issue '/set aifill 4' before starting the game to play against 3 AI players. The AI players are quite dull, though. They can't trade yet.
I can't play, where is the "New Game" menu entry?
You have to start a server and connect to it, or start the server on-the-fly from within the client connect dialog. See the Getting Started section for details.
Some menu items/toolbar buttons/popupmenu entries are disabled. I can't reach a specific command. What can I do?
This must be a bug. Please report it to the author, with a description of the actual game situation. Nevertheless you can always reach the commands by using the command line. Read the command line introduction.