rubbermaid fasttrack on cinder block where to put the screws?

On the cinder block. The mortar between the block is brittle and will break apart once you drill your hole. Over time it is more likely to loosen then putting the screw directly into the cinder block.

EDIT: To be on the safe side though, it might be worthwhile to also use concrete fasteners or anchors/inserts, depending on how much weight will be hanging on the tracks.