If people are still having issues with this, the below may work. I've been able to access our agent account several times today, albeit with a bit of toing and froing.
If you receieve a screen after logging in which shows the 'Sorry, the service is unavailable' message, go back to gov.uk/log-in-register-hmrc-online-services and click sign in again. You may get another message after doing this which says 'Sorry - agents can't use this sign in page yet' - ignore it and click the sign in link below. That takes me direct to our client account.
More bugs on the HMRC site than in an MI6 facility.