Okay...just looking at Technote 1061 document at:
http://developer.apple.com/technotes/tn/tn1061.html 

"The Open Firmware user interface ... comes up in two-machine mode.  You
can connect the two machines together using the serial ports and cable.
Start with the modem ports for simplicity.  Open Firmware defaults to the
modem port.  Use a communication application such as..."

Okay...I've only started working with Macintoshes this year, so bear with
me.

I've got a printer cable plugged into a printer port on an old Mac Classic
and into a printer port in the Macintosh Performa 6400, the computer I am
trying to install NetBSD 1.3.3 on.  I have ClarisWorks communications on
the Classic, settings at 38400/8/N/1.  I would use the modem port on the
Classic but it is in use [I have used the printer port before to send
files between these two machines], and on the 6400, there is a piece of
plastic covering my modem port [I have a COM slot modem card].  When I
boot and press Command-Option-O-F, nothing happens.

What I am assuming has happened is that Open Firmware has tried to
establish a connection over the modem port, which is being "used" in a way
by the COM slot modem.  And I can't plug in a cable to the modem port
because of the plastic covering it.

So, is it possible to remove the plastic thing?  And, I also assume the
modem would have to be removed if I did so.  Or is it possible to somehow
configure Open Firmware to use the printer port?

Thanks,

--fission