I've got a release with three CDs, each with a disc ID. When I open it in the advanced editor I naturally don't get the "add track" bit below the medium because of the disc IDs.

However when I want to add more mediums to it, they don't have the "add track" bit either. According to Kuno this might be because the HTML for the new medium gets copied from the first disc so the <div> for the "add track" bit is missing.