I am planning on overseeding my lawn this fall. On the Scotts website, it tells me to lay down a thin layer of top soil first for the seeds to root into. Does anyone know the best way to lay it down on an entire lawn? Should I get a load of black dirt in, or would it be best to buy those bags of top soil? Will it spread in a fertilizer spreader? Any help will be greatly appreciated.

Mheid, yes, by all means, a lawn spreader can effectively spread topsoil over a lawn area.
The spreader would have to be able to open itself to allow the crumbly nature of such soil to drop. Otherwise, a shovel or spade, or by hand, can be used. Just think of it like the pictures showed Johnny Appleseed strewing seeds across America....a bag attached, he walked back and forth covering the whole area.

As far as how much, just mention the area...
i.e. the length X the width in feet, and inform him what inch of layer you intend to lay over the area. Unless the area is very large, probably a a couple of bags of such topsoil will suffice. Don't layer more than 1"...more likely, 1/2" will do nicely. Then follow guidelines for the best germination.

There is no real need to pay for expensive soil...or compost. Any decent soil will do...but...more importantly, buy the best quality seed you can afford.
Much better...lousy soil...quality seed, instead of the other way around.

I'd say buy a quality loam don't short cut it and buy in bulk rather then bags .
Forget the fertilizer spreader save that for the fertilizer !
Go with the tryed and true way . Wheelbarrow , shovel , rake and lots of mussels .
If you've decent soil in place rent a tiller to break the top up and rake it smooth but now would be a wonderful time to amend the soil also.

Whether to use a spreader or not depends on which one. A drop spreader will not work, even open all the way and especially if the soil is even a bit damp. A broadcast spreader might work, depending on which one you have. I've not found the ones for a small lawn to be very good for soil, capacity and spread, and the larger ones simply cannot get moved around a small area very well.
If the turf is so thin that overseeding might be necessary, if the present turf is not what is wanted and a different seed will be spread, then the best solution is to start over and apply a good loam before seeding. Otherwise feed what you have with a good natural lawn food, after getting a good, reliable soil test to determine what is needed in what quantities, as well as the soil pH which is as important as the "fertilzier".