The forum permissions "Max file attachments" and "Max file attachment size" will need to be turned on for each user group allowed to use the file manager; likewise, "Can download attachments" will need to be turned on for groups who're allowed to download.
Additionally you'll want to make sure your attachment settings have been configured [CP -> Master Settings -> Attachments] and that you've setup your paths for attachments [CP -> Master Settings -> Paths & URLs]
