In the main navigation you will see a green magnifying glass icon. This is the search feature:
If you click on the icon, a search page will launch. Simply type in a search term (or Tag ID) and everything associated with the search term will appear.
From here you can filter by type (e.g. campaign / mailing / tag / template / content .source) or use the icons to carry out an action. For example, if you click on the edit option for a tag, this will take you to the tag wizard for that tag.