Hi, I'm trying to replace a conventional light switch with an x10 switch (model XPS3) in my 1930's-era house, but I'm having a bit of trouble.

The instructions for the new switch say:

Inspect inside the wall switch box for existence of white neutral wire(s) capped-off. If no white neutral wires exist, STOP. You CANNOT use the XPS3 unless you run a neutral wire to this box.

The instructions then call for hooking up three wires: Load, Neutral, and Line (HOT).

Inside the existing switch box, there are only two wires, white and black. I'm not really sure which is which, and which I'm missing. I turned the switch off (but left the circuit on) and tested the voltage from ground to each terminal using a multimeter. The white terminal read 120V, and the black read 0V.

For what it's worth, about 1 foot to the right of the switch box is an old (apparently decommissioned) fuse box, which looks like it has been worked on semi-recently, as it connects the old fabric-wrapped cables to more modern wires. Here's a picture

(I'm hoping that whatever wire I'm missing on the switch box is in this fuse box, so I can run the missing wire somewhat easily.)

More info which may useful:
- The lights controlled by the switch are ceiling-mounted fluorescents (3 in total).
- Full instructions for the new switch: http://www.smarthome.com/manuals/2207man.pdf
- Why I'm installing a new switch:
The switch box is at the top of the basement stairs. I'd like to be able to turn the lights off from downstairs if I leave through another exit. So I also purchased a wireless X10 remote which should let me communicate with this new switch to turn the lights off. If there is another way to accomplish this without x10 or major wiring upgrades, I'm open to it.
- "Call an electrician" is an acceptable answer.

Any idea how to proceed?