You have to get out of the tunnel, into the electric room and turn the electricity on. The boy will help guide you. After that, you will see 3 buttons at the top right corner. One of them is for saving. Type in a name to save.
Sorry, I had to go into the game to find out.
Also, the game is not very alt/tab friendly. If you need to post in hints, I would recommend you exit the game completely.