You will need a female threaded trap adapter to screw onto galv pipe. That will then accept the 1-1/2" tubular drain assembly. HD may have to sell you a PVC fem adapter and a ftg trap adapter or, use a rubber fernco with a pvc ftg female trap adapter.

What a dummy at HD

thanks...and even my time tested approach to home depot (find the oldest guy there, ask him) didn't help me.
Thanks again.

Did you clean the threads and use the compression ring in the nut?

In our old houses the PVC always screwed right onto the galvinized, no problems.

The thread sizes should be the same. I think hendricus has pointed out your problem. The threads have to really be cleaned up on the galvanized for anything to screw on there in a reasonable fashion. Of course if the threads are stripped then it't to plan #2 which someone else has suggested...a rubber Fernco.
M