That photo looks like an interlock switch. It's listed in the schematic as a DPST, but, based on what other people describe, it's physically more like a DP3T switch with some of the posts tied together. Push in and the system powers on. Pull out and the system powers on. Leave it in the "middle" and it's in an off state. From what I can tell on the schematic A/B DO NOT connect to C/D, that would be a direct short. One pole is between A & B and the other pole is between C & D. To put it simply, pop the switch and the continuity between A & B is broken. Same for C & D. The quality of my copy of the schematic is less than stellar, so it's probably best to take a multimeter and test this.
According to the schematic, it's wired in series. one brown lead goes to the AC plug, the other leads to wherever.
One blue lead leads up to the main switch which is a SPST wired in series. The other blue lead off to wherever.
In other words, according to the schematic, one of the blue leads should already be connected to the main switch. The other lead on the main switch leads off to the AC plug (with an inline fuse).
DO NOT depend on my information, the copy of the schematic I have isn't in the best quality, what with water marks and splotches all over it. I don't own a MS Pac much less any cabinet with this type of kill switch. I simply looked at the schematic as the OP should do as well.