I just help my cousin program a remote to replace a broken keyless
remote transmitter for his '93 Pathfinder.

EASY.  Save a bundle do it yourself

He got the transmitter at the dealer.

It came with instructions but it did not tell where the electronic
module is located.

On the '93 Pathfinder the module is located under the driver seat.

Remove the seat. Easy. Four bolts.

There are two modules under the carpet. One closer to the door and a
second one closer to the center console.

The one closer to the center console is the one you need to work with.
You should see a Nissan label which is
covering a hole in the casing.  Peel back the label and you should see
a button.  This is the button you use to program the remote

Put the key on the ignition. Turn to "ON" then back to "OFF" Press and
hold the programing button. The siren will beep four times, follow by
five beeps. Release the programing button. The siren will start beeping
on one second intervals. After any beep you can press the large
arm/disarm button on your new remote. The siren will emit a
confirmation tone. You can program up to 4 remotes. Now turn ignition
key to "ON" then "OFF" to exit the programing mode.

DONE