We're seeing reports from some blog owners, who purchased custom domains through Blogger / Google, who are unable to access the registrars zone editor through Google Apps.
Owners of blogs published to custom domains, where the domains were purchased through "Buy a domain", need to use the registrars zone editor to maintain their domains. In the past, the zone editor could be accessed from a link in the "Advanced DNS settings" wizard in the "Admin console" / Google Apps dashboard.
The "Advanced DNS settings" wizard provides important tokens, to let the owner of a domain, purchased through Blogger / Google, interact with the registrar. Besides the tokens, there is a link, captioned "Sign in to DNS console", which should lead directly to the sign in screen provided by either eNom or GoDaddy.
Right now, the "Sign in to DNS console" link leads back to the "Admin console" dashboard. This does not help the domain owner.
Having discussed this issue with a Google Apps Top Contributor, it appears that the only way to login, right now, is using the correct registrar login screen. The link must be properly selected, according to the registrar who hosts the domain.
The domain owner will still have to access the "Advanced DNS settings" wizard, to retrieve the "Sign-in name" and "Password" tokens, which are unique to each domain. The tokens, once retrieved, will be used on the appropriate eNom / GoDaddy login screen, as linked above.