Your Turbine account by itself does not allow you to play any of their games. The account needs a subscription to the game, which allows you to log in and play. Each Turbine account can have one or more subscriptions on it, and you can mix and match them. You need a LOTRO subscription to play LOTRO, a DDO subscription to play DDO, and an AC2 subscription to play AC2. Note that from the account level, the term "subscription" just means sub-account, and does not mean you need to be paying for the game. It's a holdover term from the pay-to-play days.
My guess is that you went into DDO first. To add another subscription to the account you will need to contact Account Management. They will provide you with a product key that can be added to the existing Turbine account. Best bet would be to call them, as they should be able to take care of it while you're on the phone. Sending in a ticket normally takes 1-2 business days. Since there was a recent expansion release, ticket times may be much longer for a while.
"Everyone's got one. Better to be a smart one than a dumb one." - me
"It's people like you that [I]think[/I] you know everything that give those of us that [I]do[/I] a bad name." - me
"Eat, drink, and be Merry, 'cause Pippin was a fool of a Took."