1) Load previously untagged track(s) that have tags into Picard
2) From left hand pane select either an individual track or a cluster
3) Click the "Lookup" button at bottom of left-hand pane

Expect search to be initiated on website, however it just loads an empty form:

http://musicbrainz.org/taglookup?tport=8000&artist=Eagles&release=Hell%20Freezes%20Over&track=Desperado&tracknum=15&duration=257836&filename=&puid=

Looking at the web form, it seems it's expecting the request parameters to be prefixed with "tag-lookup.":

http://musicbrainz.org/taglookup?tport=8000&tag-lookup.artist=Eagles&tag-lookup.release=Hell%20Freezes%20Over&tag-lookup.track=Desperado&tag-lookup.tracknum=15&tag-lookup.duration=257836&tag-lookup.filename=hh&tag-lookup.puid=