To create a new tab, you will first of all need to ensure you are on the dashboard and have created a twitter list (please see the "How to create a twitter list" guide).
At the top of the dashboard, click "configure tabs", then "add a new tab" and give it a name.
Click add a new personalised stream
Ensure "twitter list" is selected, and then choose the accounts at the bottom that the tab should display for.
Just below "Twitter list" click the "select twitter list" button and choose your account.
Select the relevant list using the select button.
Close the pop up box and then click the name of the tab you have created just above the columns.
This will then display your personalised twitter stream.