I have this black box (actually it's off-white :) that hooks up to an
  optical fiber cable.  There is a jack, labeled LAN, that a regular
  ethernet wire plugs into.  The instructions say to plug your computer
  (Windoze of course) into that.

That doesn't sound like DSL; it sounds like Verizon FIOS.  A friend of
mine has that, and apparently one just does DHCP to the box.

  Another idea I had was to plug a WindowsXP machine into the optical-
  ethernet converter black box, and then see if I can find the missing
  information that I need to get NetBSD to connect by looking at the
  WindowsXP network configuration.  Anyone have a idea what things I
  should look for if I do try this?

Get a hub, and plug in both a NetBSD box and windows.  Before powering
on windows, do

  tcpdump -s1500 -wWINDOWS -i fxp0

(or whatever your interface is)

and get the windows box on the net with the provided tools.  Then go
back and look at the tcpdump to see what it did with "tcpdump -r
WINDOWS".  The point of saving to a file is to avoid having to choose
filters on the fly; you can run tcpdump with different filters later
after capturing the raw data.

[This really belongs on netbsd-help, not port-i386.] 

-- 
        Greg Troxel <gdt@ir.bbn.com>