Thank you for your post.
Open the file, pull down the File menu and select "Sharing -> Instant Web Publishing". Turn on Instant Web Publishing (which I assume is already on). In the bottom left corner is a listing of currently open FileMaker Pro files. Click on the file that you want to share via Instant Web Publishing, and on the right side, make sure the access is set to All Users. Click OK, and now the external users should be able to see the file under Instant Web Publishing.