User reported that pressing "Add Disc" doesn't do anything (I attempted with manual tracklist) on
http://musicbrainz.org/release/55c91ec5-4326-47a3-a6c9-5ced1c12ddd0/edit
(Basic tracklist editor)

http://forums.musicbrainz.org/viewtopic.php?id=2908

After lots of trial and error I seemed to work out that it works OK if you load the tracklist for the last disc prior to trying to add the disc. Appears to be the case for both Basic and Advanced tracklist editor.

After you've pressing "Add disc" you can also no longer load the final medium. (Firefox 4 just shows "Now loading tracklist..." spinning, but no web requests are made - and no errors in JS console)

It appears to only be a problem for releases which are too big for the release editor to initially load all the tracklists.