How should I waterproof a concrete roof?
You're probably looking for an elastomeric roof coating. 

Primarily, you'll need to clean the roof, repair and fill any cracks/gaps/etc and remove any of the peeling drylok, apply an appropriate primer meeting the manufacturer's suggestions, then apply at least two coats of the roof coating. It would also be recommended that any sloping/drainage issues be addressed as well in order to avoid heavy pooling, where possible.

As usual, cheaper is rarely better when it comes to roof coatings. A white coating tends to last longer and reduce roof temperatures (which should also help with interior temperatures).