The message when trying to attach a completely new disc ID says:

"There are currently no discs in MusicBrainz associated with the information you provided. You can search for the disc you are looking for using the forms below, or you may add a new release to MusicBrainz"

But there's no release-adding button. That's intentional: we want users to search and confirm the release is not there before they add it. But it's very confusing at the moment.

The message when trying to attach a disc ID we already have says:

"We found discs matching the information you requested, listed below. If none of these are the release you are looking for, you may search using the form below in order to attach this disc to another MusicBrainz release."

That one doesn't even mention you can also add a new release if it's not found.

We should probably make the two consistent, respectively:

"You can search using the form below in order to attach this disc to another MusicBrainz release, or to add a new one if the search shows it is missing from the database."

"If none of these are the release you are looking for, you can search using the form below in order to attach this disc to another MusicBrainz release, or to add a new one if the search shows it is missing from the database."