By default, the program starts the game with both sides controlled by the mouse. If you wish to play against the computer, you can select which side the computer plays (selecting one side automatically de-selects the other side - you are not allowed to let the computer play against itself, as this would be too farcical) via the Settings menu.
There are two versions of the computer player. The Version 1 opponent (the default), is slow and week, but is built-in to the program on all systems. The Version 2 opponent is much faster, and consequently stronger. However, it may not immediately be available on your machine, as it requires compiling separately. If it is available, then this is the version you should use. All new features of the computer opponent will apply to the Version 2 opponent only.
It is worth pointing out that the speed of either version depends on the handicap which is being used. A Free King and a Dragon King is the fastest handicap, and Three Lions is the slowest. The speed difference is considerable.
That this is so is simply due to the fact that the fewer pieces there are on the board, the fewer moves the computer has to consider. The fact that Three Lions is slower then an even game, is due to the Lion having many more possible moves than a Kylin.
In the Version 2 opponent, the actual time taken depends upon the time selection parameters. Therefore the speed difference translates into a difference of depth-of-search, and consequently, strength.