:ba: … nevermind-d-d-d-d …
MBF (acronym): Momentary Brain-Fart.
… or, to put it another way, “why you should not attempt to use Blender at odd hours, even though you often do.”
I suppose it’s worthwhile to “fess up” to the details of my momentary stupidity for the benefit of the next poor soul. (Yes, you! Hi, there! I’m talking to you!) It’s quite easy to do, and not quite intuitive.
When you open a library file (and of course, any blend-file can serve as a library …) you will see a breakdown of everything that’s in it … including “cameras,” “lamps,” “meshes,” and so on. But these, really, are consistuent parts of what you probably want to be linking to … objects.
It’s quite easy to see “how Blender sees this” once you’ve done the linking … to objects … as Horseman pointed out. Switch to Outliner view, and there are the three objects: Camera, Lamp, and Cube. Now, expand each of those three nodes. Beneath them are identically-named “constituent parts,” which are owned by the linked objects.
As the Blender docs say, you probably then want to “Make Proxy,” and when you do this you’ll see the “_proxy” objects at the top of the Outliner hierarchy, with two things underneath each one. (One of them being the linked object.)
While you can link to “anything,” AFAIK, such as a “Mesh,” only Objects have a physical presence in 3D space. (You’ll have to make the association with an Object yourself before you can see them.) “Groups” are especially handy to link to.
Sigh … but, 'tis better to fess-up to oopsies, just in case thereby you won’t follow suit.