What's the best way to waterproof a concrete structure for a tropical climate?
I would use the HRWR admixture to help seal the concrete. Alone, it might not be the best roofing solution, but it's well worth the minor cost for the extra protection.

The biggest problem that I heard was foot traffic. Normally I would recommend EPDM, but I'm not so sure about foot traffic. This sounds more like a terrace, in which case, silicone cement is most commonly used.

However, something like a layer of asphalt would be more durable. This could be layered over steel. I would consider using coal tar (sealer) on top of the asphalt (which works better than bitumen/petrol tar) for shedding water. However, using coal tar is against roofing codes (causes cancer) in most of the world. 

But one alternative that was not mentioned would be a green or living roof. I don't really know much about the materials involved, but in a tropical climate, it might be best to just go with the flow (in a sense). Fighting the elements can sometimes be harder than working with them.