Beth,
In the most recent v3.1, we add a bunch of new fields and refined what we're able to import. Unfortunately, not every field that you see in a user's Facebook profile is able to be imported. Facebook does some shorthand on its site so that a user can enter a zip code, and it will look up the user's location from that. However, all it will let us/JFBConnect import is that location that it determines.
You can see a full listing of the fields we can import from the link below. If you search for "location", you'll see that what it returns is:
A JSON object containing name and id
Regardless of what a JSON object is, the important part is that it only returns the users location in a city, state format and Facebook's internal ID for that location (which doesn't have much value).
developers.facebook.com/docs/reference/api/user/
Sorry for the inconvenience, and if there's a way we can improve this, or if you know of a site that's doing things differently, let us know. There are tools out there to look up zip codes based on a city/state location. That's about the best option I can suggest for you to get the user's zip code.
For your information, long ago in the 2.x series of JFBConnect we actually could get the zip code, but Facebook changed that functionality back in July.
Hope this helps, and if there's anything we can do, let us know.