Find the block "Game Starter" under blocks tab, put it on the ground, make sure that the ground is flat.
Or you could also craft the block in survival mode, in this way:
dirt stone dirt
stone dirt stone
dirt stone dirt
Then right click the block, a chess board should be set up with chess pieces on it.
You can right click a chess piece to pick it up, right click again to put it down.
And right click the red cross block named "Destroy Button" on one side of the board to erase the chess board.
Have fun!
ps: Since there is no AI created, this mod is only playable on servers. Well, unless you liked playing chess with yourself.
*The models are created with Techne*