Assuming you had the attachments in the file system in vB, then the issue is that the path you're entering to the files is wrong or the files can't be read for some reason. Unfortunately, it's hard for us to provide specific support for a custom path that would depend heavily on your server configuration and location choices you may have made. I'd recommend entering the file path to the attachments folder relative to the root XenForo directory (use ../ to go up a level).