There is no enable button. PM's in vBulletin don't have attachments.
I'll look to see if someone has a hack for it though, but be warned that as it won't be core vBulletin code I cannot guarantee that attachments remain private. (By this I mean that the attachment system hasn't been designed to be private and so it may be possible to access attachments even though the PM itself isn't accessible.)
There is no enable button. PM's in vBulletin don't have attachments.
I'll look to see if someone has a hack for it though, but be warned that as it won't be core vBulletin code I cannot guarantee that attachments remain private. (By this I mean that the attachment system hasn't been designed to be private and so it may be possible to access attachments even though the PM itself isn't accessible.)