Thanks for the many helpful suggestions. Here's where the painful
path has taken me so far:

Jeff at R-Track kindly provided the USB driver by email, and I
installed it by hand. I also installed the similar FTDI driver
recommended by Darryl. Neither worked. I did see the dancing mouse
cursor issue mentioned by some in other threads. No amount of
rebooting or reinstalling made a difference.

The serial-to-usb adapter I ordered has a large housing for the
electronics integrated with the serial plug, too large to fit onto the
back of the 302. So I ordered a serial extension cable, it arrived a
few days ago. Today, I finally got that hooked up, and the laptop
recognized it and assigned it as port 17. I fired up the Cambridge
utility, set the port to 'other' and number 17, and... nothing. I
finally tried remapping the adapter port assignment to port 1 using
the adapter assistant software, and set that in the Cambridge utility,
and... SUCCESS! Perhaps when the Cambridge utility software was
written nobody ever thought there might be so many ports available,
and it's out of the undocumented usable range?

So, again, thanks for the help. I agree the usb mod on the 302 is
useless, but at least I can now set the aircraft type and
identification (the only two things that must be done using the
Cambridge utility, with no other work-around option).

By the way, I also now have one of the new LX Nano flight recorders,
absolutely love it, so simple to use, no connection to the glider
required at all, connects effortlessly to any laptop for declarations,
just turn it on and put it in your shirt pocket -- really makes the
302 seem like an antique as a recorder, though of course I still value
all of the flight computer and variometer functions.

NG