This is a problem. When you rip a CD, a special (semi-)unique code is generated for the CD, and this is used to do the metadata lookup. And MC uses this metadata not only for the physical CD entry in the MC Library (which you typically don't see), but the metadata is also propagated to the tracks that are created.
Without this unique key, there's nothing to use for look-ups. It is possible to create a system that looks-up by Album and Artist, it gets more complicated because of all the releases for an album, so you might also need a Date, and a Release version, number of tracks, etc. A GUI dialog could be presented with choices, but that doesn't exist yet. It's more complicated than most users want to deal with, and tends to muddy up the re-submitted release data in any event.
You can insert the CD and MC will do the lookups. Then you can Copy the tags from the Physical CD entry and Paste Tags to the ripped tracks.
There are other tools that will do acoustic signature matching, and will populate the track meta-data from that. They do a pretty good job, but aren't always perfect.
I've written a couple of scripts that grab data from amg, and one in progress, from discogs, but both of these also need an ID (which you supply):
http://yabb.jriver.com/interact/index.php?topic=84268.0 http://yabb.jriver.com/interact/index.php?topic=86465.msg594662#msg594662Still, they are easier than entering all the metadata in manually, and as a bonus, you can get the extra info not available via MC (Composer, extra credts, etc.). I have another
general tool that has a scriptlet which does acoustic signature look-up, but I haven't made the scriptlet available yet.