What type of underlayment should I use for an above ground pool?
Per request, I'll expand my comment into an answer:

as for the joints in the concrete, I'd use polymeric sand (typically used for locking pavers). 

Then the key is to create some sort of smooth pad in between the concrete and the bottom of the pool. Some options:


roofing felt (tar paper).
Ice and Shield roofing membrane
rubber pond liner
epoxy (if it's not terribly rough, a few coats of epoxy concrete sealer might smooth it out)