Hi Chris,
Unfortunately, it was almost certainly a mistake to delete the forum.
There's no ZIP file involved in creating a new forum. What are you doing? If you are trying to restore a forum from the file that Nabble sent you when you deleted your forum, then the thing to do is send the file back to Nabble who will use it to restore your forum. All you can do is create a new forum under a new name.
It might have been worthwhile asking your users to send you a screen dump of exactly what they do see on their screen. If there really is a problem then it will help Nabble diagnose it. I've known people say "I can't see the captcha" when what they mean is "I can't read the captcha" In that case the user had failed to work out that they all contain five characters, a mix of upper case letters and numbers (no lower case or punctuation) and they were repeatedly typing the wrong code.
It could also be worthwhile asking exactly what platform/browsers those with problems are using and make a test at:
http://browsershots.orga remarkably useful tool when people tell you a page doesn't display as expected on their hardware/software.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.