Hey Paul,
Having replaced a McClicky in my Malkoff Hound Dog tailcap, I'll chime in. Not sure how much different the Hound Dog and MDC tailcaps are, but in the HoundDog tailcap, the McClicky unit itself is held in the tailcap by an o-ring. I think the "replacement switch" you're referring to is just the McClicky unit itself.
In the Surefire Z41, it requires a little brass fitting ring that screws into the Surefire tailcap's threads. It holds the McClicky unit in place. There's some threads on here that discuss how to put the McClicky in the Surefire tailcap, I can't locate a good one right now. I think Oveready had a guide on their site at some point.
1) You have to punch out the pin holding the old LOTC guts in place, unscrew the disk in there. This process ruins the old rubber Surefire switch boot, so you need a new one! I recommend using a stiff press version.
2) Place your new switchboot in place, drop in the McClicky, and then screw down the brass fitting ring, making SURE the McClicky spring is square and secured in the boot with the spring cleanly sticking up toward your batteries. Tighten it down securely but don't overtighten.
I just replaced one in Z41, Lumens Factory dealer "Shastadeals" on the 'bay is a great seller for these. If you search for McClicky upgrade kit you'll see the switch and the brass ring.