Found this in the FAQ: A Tour of Duty Ticket gives you access to Mann Up Mode, and can be purchased from the Mann Co. Store for $0.99.
Valve is making us pay just to track our stats!
I appreciate that it is a very small cost, but my parents refuse to add money to my Steam account. So some, like me, will forever be unable to track our MvM stats...
BTW: I'm not F2P