1) Go to your garage or LBS (local bike shop) to find a taco'ed bike wheel from which you can clip the hub. Make sure it fits the bike you intend to haul around on top of your car, and find a a quick release skewer that will fit.
2) You better take a break: this has been pretty strenuous so far. Have a refreshing beverage.
3) Get an angle grinder, metal file, gritty sand paper, or use your teeth to make flat spots on the hub where it will rest on the roof rack's cross-bars. If you have Yakima bars (round) instead of Thule (rectangular), then make your flat spots convex to roughly fit the radius of the bar.
4) Go to your garage or LHS (local hardware store) and pick up a U-bolt that will fit snugly over your hub and cross-bar. If the threaded portions of the U-bolt are too long, you can just hack them off with a saw at later.
5) Fasten the hub to your bars with the U-bolt, and tighten it down.
6) Throw your expensive carbon-fiber mountain bike up there.
7) Cruise down the interstate
8) Go back to where your bike fell off and pick up the pieces off the road.
8) Now, back at home, adjust your rear bar so that your bike's back wheel lands on/near it and then get a nylon strap to secure it.
9) Have another refreshing beverage and admire your work.
