I can't figure out how I to create a relationship between two different files, I only see an opportunity to link between tables in one file.

You need to define a File Reference, then you can create the relationship.

That's what I thought, even though the online help didn't tell me anything about it, so that's what I did before posting here.

I have a file reference to File #2 in File #1, (I also put a file reference in File #2 to File #1) but File #1 doesn't offer me File #2's tables or fields.

(I'm trying to set up the eAuthorize black box from Waves in Motion)

You have to set up a relationship after the file reference. Go to define database, and then click on relationships.

I feel better now, it seems I'm doing everything right, but still it doesn't work. I contacted Waves in Motion support this morning, hopefully they'll get back to me soon.