Create a server... nothing to subscribe

[Harry Putnam <reader@newsguy.com>]

I think I'm following directions in the manual at 
  (info "(gnus)Creating a Virtual Server")


  (nnml "b"
        (nnml-directory "~/Mail2")
        (nnml-active-file "~/Mail2/active"))

But when I  open the server hoping to maybe subscribe to something in
the directory, nothing is shown at all.

The directories under ~/Mail2 are directories with numbered messages
inside.  But have never been part of a gnus backend before.

Gnus doesn't seem able to create an `active' file so I'm offering one
with just a list of the directories under ~/Mail2

 cat ~/Mail2/active

    [...]
  
    all.bk
    bogo_spam.bk
    bogo_unsure.bk
    mail_cron.bk
    mail_fw.bk
    mail_rsnap.bk
    mail_rsync.bk
  
    [...]

Should gnus be able format the active file?  Do I need to add the
notations about how many messages etc myself?

If that is the case, I've forgotten what the notation really means.

  Bobbie 71 33 y

And not doing much of a job finding it in the manual.  The index item
for `active' here:
  (info "(gnus)The Active File")

Doesn't appear to go into that kind of detail.