1) Am I correct in thinking that the descriptor_surf is the operator I need ?
There are several types of Interest Points detectors, you can combine them with Local features extractor with PoI. This could be one of approaches, but nobody knows if it will work unless you will try it.
2) Why are all the values zero, do I need to pre-processed the images or do some other conversion before running it?
It it wrongly implemented in this old version. It needs defined widow RoI with some scale, but I can't now find any operator which generates these windows.