The question mark means that the object is no longer connected to the server.
AFAIK the _admin volume object isn't really necessary in edirectory.
You can add the _Admin volume to the directory with consoleone, just like any ordinary volume.
In the container where your server resides, Add a volume, point to the host server, select the _admin volume and name it SERVERNAME_NSS_ADMIN.
[Root] should be a trustee of this object, with Read rights to the properties "Host Resource Name" and "Host Server"