There are floor levelers but you need to know the 'depth' of your floor's drop. Some levelers can only be 1" thick. Once you've determined what it will take to level your floor, you can go shopping for Floor Levelers.
As far as moisture, you need to determine the extent of your moisture. Cut a bunch of 2' x 2' pieces of plastic, tape ALL 4 SIDES COMPLETELY DOWN to the floor (no edge exposed), and wait 2-3 days. Then, carefully rip up the squares and determine if the moisture on the underside is more like condensation or 'pooling'. If it's the former, you're OK. If it's the later, you'll need to dehumidify. Of course, the 'denser' the moisture problem, the less options you have for material like glue and such. You may even need an indoor/outdoor carpet that can 'survive' higher-than-normal levels of moisture.
My best to ya and hope this helps.
Jay J -Moderator
As for a moisture barrier, I have also used UGL Drylock paint, a waterproofing paint. It is designed for walls, not floors, but should be o.k. if covered up. The oil-based UGl paint states on the label to not use it under carpeting, but the latex, water-based UGL paint has not such warning, so I would use it.
Finally, you can also use builder's felt. Trowel down a layer of asphalt mastic, and allow it to dry. Unroll 15-pound builder's felt on the floor, overlapping the edges by 4 inches. Trowel a second coat of mastic on top of it, and then lay a second layer of felt on top of that, with the same sized seams running parallel to, but between, the first layer's seams. Doing so should also help level a slightly unlevel floor.