If you import from FreeDB (e.g. http://musicbrainz.org/freedb/misc/af0a920c ) the artist name is put into the "artist as credited" field instead of into the "artist in MusicBrainz" field.
This means that the artist lookup box is disabled and you have to open the artist credit editor, copy the contents of the artist as credited field and paste it into the artist in MusicBrainz field to do a lookup.