I work with FrontPage, and while it has received a lot of criticism, I believe it to be a good HTML editor. But, FrontPage can act like a spoiled rotten child from time to time.
If you are downloading pages, the extensions are in tact, and you are all set. But only if you are using FrontPage to download the files. If you are using an FTP to put HTML docs on the server, you will need to have FrontPage extensions installed on the server to download direct in FrontPage. Now, assuming you have the extensions installed. (contact the web host provider for the extensions)
Everything you do in FrontPage, any changes, must be done in the program to keep the extensions intact. If you have moved a file, renamed a file....anything. If it was not done in FrontPage, you have broken the extensions and they need to be reset. The web hosing provider can help you with this. Also, don't move things around when you are in the transfer screen, FrontPage really does not like this. The more in sequence the website is with FrontPage, the better chance you have with maintaining the extensions. The message you are receiving is telling you that the FrontPage extensions have been broken, and you need to restore them. This cannot be done in frontpage, but must be done at the server. It's usually fairly simple. You simply eliminate the FrontPage extensions on the server, then wait about 5 min, and then reactivate them. Also, you should not be using a FTP client and FrontPage together, this will break the extensions, as well.
Yup, she's a cranky one, good luck.