The program is a chess database system. You can use it to play against the computer but you specifically have to choose the Play against computer menu option.
Not sure accurate the following is but should be helpful to you in figuring it out.
Scidvspc.sourceforge.net/doc/ComputerGame.htm
I just read that link a little better and it probably doesn't help much 
I'm sure someone else can clarify, but if not, I'll post something later when I get the program in front of me.
OK, after you have your position set up you have to go to the Play menu option. In general, you'll probably use the UCI option there. Then choose the checkbox to start from the current position.
You'll also want to make sure the side you are playing is at the bottom.
Hi All,
This is the first time trying SCID vs PC app. It is installed on Linux machine. I am setting up board to analyse some end games. After setting up the board, when I make my move, the computer does not make its. I am not able to figure out if there is some specific option I need to set so that the game can progress. Please assist.