To try out the collection functionality go to http://test.musicbrainz.org and sign in. If you already have a MusicBrainz account you can use that one, but all passwords are set to mb. In the top menu there is a tab named collection. In the pages under that tab you can access collection functionality. For info on how to add releases to your collection se the Adding releases to collection heading.
Adding releases to collection
Releases can be added to your collection either from the website or by using a tool which tells the server which releases you have in a specified directory on your computer. This requires the releases to be tagged.
Add a release from the website by going to the release page of the release to be added, and click the Add to collection link.
You select which artists to see missing releases of, and of which to be notified about new releases. This is done by going to the artist's page and clicking the Display missing releases or Notify on new releases link. From the artist page you can also unselect displaying missing relases or notifications on new releases. This can also be done from the artists page under the Collection tab. There you can see an overview of selected artists.