There are a couple of ways to search the directory:
Use the Search People box
This quickest way is to use the Search People box on the Outlook ribbon. Simply begin typing a person's email address or name and Outlook will return matches from the directory. This search only matches against email address and name.
Use search box in the People pane.
- Choose People at the bottom of the screen.
- Above the contacts list, type someone's name or email address in the Search Contacts box.
- This search will match a person's name, email address, phone numbers, and more.
For more details, see Microsoft's Find people and contacts help article.