I have a KS200MP2 switch that I'd like to use to control what I think is a 3-way configuration. There are two switches that control the same light at the top of the stairs.

When I connect it, the switch powers the light on and off rapidly. Is this because this switch does not work in this configuration?

It's also a really old house. While it has a neutral wire, none of the wires are colored. I only know the wire is neutral because of markings on the old light switch. There also is no ground.

I used a probe to determine which wire is hot, but does it even matter with this switch, as long as it's connected to one of the line/load wires?