What finish can I put on the pallet floor to fill cracks

You need to see if you can first remove the dirt. The depth of the footers will determine that. If you can remove dirt, you'd want to remove enough dirt so that you can put down a gravel base, compact it, then pour a new slab.

If you can't remove the dirt, you might be able to pour a raised concrete slab.

If you want to go super-cheap, lay down and seal a plastic barrier above the dirt just as you'd do in a crawl space.

Correct, dry shake. Or more specifically dry shake color hardener. As you say, it is essentially cement and colorant. You might just search video sites for application instructions, such as http://www.youtube.com/results?search_query=dry+shake+color+hardener+application. Incidentally, techniques that achieve similar effects to cured concrete are concrete dyes and concrete acid stains.