Just helped my son complete the Brain Game (MK112). Short story, it did not work. EE 101, check the assembly. Looks very good. Check the power. Ah Ha! No power. Check each battery. All are good. Add each battery in order. All add up! The wire from the negative terminal to the board. Solid! Hmmmm! Pulled out the battery in the middle and measured continuity between the spring that is on the other side of the negative output of the battery holder. No connection! Tried compressing the riviet press fit, but no dice. Soldered the spring to the rivet. No connection. Finally soldered directly all the way to the wire connection. That fixed it! It was almost as if the material on the solder lug on the negative side of the holder was not a conductor. After bridging it all together with solder, everything was good. My son and I love the game. It works perfectly. BTW - did find that the schematic is miss-labeled per the circuit board.
Bottom line - What I should have done, was run the wire from the board, all the way through the battery holder lug, and up through the rivet hole and solder directly to the battery spring. Have done a lot of these with my son, and this is the first defective battery holder, but at least I have finally learned to answer question #1, “is it plugged in?”