It depends what you want it to do, do you want it to only affect one pickup or all of them? A schematic would help everyone greatly.
If you want it as a master mid control then try it just before the master volume control. So take the wire leading to the input of the volume pot, remove it from the master volume and connect it to the INPUT of the mid control. Take the OUTPUT of the mid control and connect that to the INPUT lug of the master volume pot. So you are effectively slotting it in before the master volume. You could also try slotting it in after the master volume, I imagine this will make it less efficient//conventional but I'm assuming thats what you are going for since you're adding ad passive mid control to a strat. Perhaps a switch to instantly test the difference between the 2 locations of the mid control?
On a side note, if its passive then it wont "boost" the mids, at most it will cut the treble and bass so it SEEMS like it has more mids. And you might want to think of putting in a DPDT to completely isolate the mid control from the circuit because it could drain the tone a bit if its not very efficient, but again, that depends on what this midrange control is actually made up of.