Here's a very simple example of what I'm trying to do:

I've created a test application with two forms. Form 1 has a single dropdown box with three choices in it. On Form 2, I'm trying to create a lookup field where the lookup relation should be the dropdown on Form 1. I can't however choose the dropdown field from Form 1 -- it simply isn't an option.

Am I doing something wrong, or is this not supported? Is there another way to accomplish the same thing?