I think it’s been down for a bit, but Rowland Cox posted this a few minutes ago:
Greetings!
The Account Center is currently offline. We anticipate that it will be back online no later than 3:30pm EDT. Please visit UOHerald.com for for updates on the Account Center maintenance today.
Thank you for your patience today.
Hopefully this will fix some issues people were having.
via UO Herald
They still should post tutorials about using the account management center.