Thank you for your post.
It sounds like the Zip codes being looked up are in a different file. You will need to open this file when you open your working file. The problem currently is that the Zip code file cannot be found. If you moved your original file, this may be why FileMaker cannot find your Zip Code file.
Once you find your file, you may want to consider moving the Zip Code information into the same table as your working file. This way, you won't have to worry about where the other file is located.