My MK131 does not work and I need info on step 11

I have your KM131 kit. After completing the kit it does not work. Your instruction sheet step is not clear as to where the center row of cathodes go. One picture shows them in the back holes and one has them in the front holes. Also you have to watch the polarity. on the holes one side is marked white and one is green. Which one is + and which one is -? Also on the cathode which is + the long wire or the short wire?

The lights are called LEDs, the cathode is the negative side of the led (short leg). And thus the long leg is the +. The leds are also not completely round, so the form should match the pattern on the PCB (the board).
The lights are placed on both sides of the board. And yes like explained above the polarity is important.
Try to finish the board. If it still does not work i suggest to upload some pictures so that it can be checked.
Hope it helps.