Thank you for your post.
Try creating a new user account, login to the new user account, launch FileMaker Pro 10 and create a new database file with one table and one text field. Then, go into Sharing and try turning on Instant Web Publishing, and click OK. If that works, then go back into Sharing via Instant Web Publishing, select the database file, set access for All users, and click OK. If both of these work, then we know FileMaker Pro is installed correctly. If either fail, then reinstall FileMaker Pro.
Return to your original user account, and create a new database file with one table and one Text field. Turn on Instant Web Publishing and click OK. If this works, go back into Sharing -> Instant Web Publishing, select the file, and set access for All Users and click OK. If this fails, the issue is in the current user account. If this works, then the issue is with the original database file.
Keep me updated with any progress.