When you first run Tweetdeck it should prompt you for your twitter username and password. (If it doesn’t, look in the right hand corner of the Tweetdeck window for the spanner icon and click on that):
A settings window will appear in the center of your screen:
Click on the tab on the left hand side for Accounts:
Enter your account details:
You’ll see the account appear in the acccount list below:
(If you want, you can add your Facebook and LinkedIn accounts too, as well as add more Twitter accounts.)
[...] Tweetdeck: Setting up your Account [...]