This is going all over the place a little bit... but a few short answers that may help clear things up a little bit:
You can manually drag the file from the left hand side pane in Picard (a 'resting' place for your files) onto the relevant track in the MusicBrainz database release (the gold/silver releases on the right hand side). Then it will be tagged using the data from the track in the database that you dropped it onto.
This means that we don't have the AcoustID in our database/it hasn't found a match.
Note that Lookup:
Uses your existing tags to find a match. If your files already have some existing tags, like track numbers and an album name, the best way to do it is to hit 'cluster', and then hit 'lookup' on the cluster/ the whole album.
Uses the actual audio data on the file, and tries to find the same audio data in our AcoustID database. If nobody's submitted the data for that track, it's not going to find it. Note that this method ignores all of your existing tags, making it much more likely that a song from an album is going to be put into a Various Artists release, and so on. But it's very useful if you have no information for a track and need to find out what it is, or similar situations.
I'd actually be keen to do this, as it'll save everyone a lot of time!
You can find me by searching for aerozol1 or try Simon Hartman, with a picture of a kiwifruit. I'm on +12 time but it should work out.