This is done via a new Tags tab in the InfoCenter.
Click the tab to display a form for managing tags on the account.
Click + Add Tag. This displays a Tag Selector. Any tags created via MANAGE > Account Tags will be available for selection.
Select the applicable tag and then click Add Tag.
Below shows the tag added.
Once added, you can remove the tag from the account as needed by clicking Manage Tags and then clicking the red delete (X) icon.
