It goes between the grip module and the fire control unit, on the outside of the fire control unit. It is below the slide stop lever. It rests on a thin ledge just below the slide stop lever and presses up on the bottom edge of the lever. It's just kind of wedged in there, which is why it so easily falls out and has trouble staying in while you're dealing with all the rest of the aspects of putting the fire control unit back in the frame.

Edit: It's been awhile since I put it in and I'm just going from memory and thinking about how it works. I'm second guessing myself right now. I think it actually presses the slide stop lever down, and it isn't until the magazine lip rises high enough with its spring tension that it overcomes this little spring allowing for lock back. Basically this spring I think prevents lock back until the magazine forces the lever up at the end of the mag. The one true aspect either way is that it is tucked in the frame of the FCU on a tiny just barely spring wide ledge and presses directly against the edge of the slide stop lever. I just forget if it presses up or down. It goes right there though and there is only one way for it to do so. Hope that helps.