Continuing the discussion from Support for displaying similar releases in Picard:
@rahulr came up with the problem on how to properly load and use arbitrary releases from musicbrainz.org in Picard. As I think this is useful for everyone I want to detail this a bit. It is also somewhat explained as part of the Picards Basics documentation.
Some basics first 
In Picard you have two panes showing the files and releases to tag. Things listed in the left pane are actual files that are not yet matched to any MB releases. On the right pane you see primarily releases loaded from MB with the tracks below. The tracks can have one or more files attached, this is indicated by the symbol in front of the tracks. The basic symbols are:
This is just a track, no file is attached to it
A colored rectangle indicates that a file has been matched to the track. The color and fill state of the rectangle indicates how good the existing metadata of the file matches the track metadata from MB.
Once you select the file and click save Picard will attempt to save the file. If you save many files at once this will take a while, a grey icon indicates the file has been queued up for saving.
Once a file has been saved this green check mark will be shown.
This error sign indicates something went wrong while saving the file. Take a look at Help -> View Error/Debug log... to get more details.
Tag your local files
No, definitely you don't have to change the tags manually. As you initially have your local files in the left pane the goal is now to load the proper releases from MB.org into the right pane and attach your local files to them. This can be done semi-automatically with "Scan" or "Lookup", both actions will load data from MB.org and try to match your local files to it (Scan uses acoustic fingerprinting, lookup uses existing metadata).
If that doesn't work you can always start a search on MB.org directly from within Picard by using either the "Lookup in browser" button or by using the search field in the upper right corner . Once you have done that you can browse MB.org however you want and also use the search functions on the website. Next to each release and track there will be that green tagger button which will load that release / track into Picard in the right pane. You can then attach your local files to the loaded releases by dragging them with your mouse.
- This should probably be added to the Picard documentation in more detail, but for now this has to do to give some context.
- My advice is always to start the search from within Picard, that will enable the "browser integration" and trigger MB.org to show the green tagger buttons. For the technical curious: If you want you can also attach the
tport=8000 parameter to any musicbrainz.org URL, e.g. just
http://musicbrainz.org/?tport=8000. 8000 in this case is the default port Picard is listening on. The port used can be configured in Picard under Options -> Advanced -> Network. If you have multiple instances of Picard running the additional instances will use a new port by increasing the default port number by 1, e.g. 8001.