They probably took the address from the caller and looked for the plane near there. Once finding it, they likely looked around the area for a guy with an RC remote.
Of course they could have also used a radio direction finder.
Hmm but he was flying over a huge area - from Brooklyn Bridge, Statue of Liberty, apartment buildings on the waters-edge, etc.
I'm wondering if they used radio direction finder - but a) would they have that equipment to hand and b) how would they know what frequency to be looking on?
Of course they could have also used a radio direction finder.