You've built your audience within Audiences. Now you can select it on the Criteria tab to associate it with your campaign.
Search for an Audience
Use the Search filters to locate your Audience. From the first drop-down list choose Audience Name, Description, Tags, Created By, Created On, Modified By, Modified On or Property(s). Choose a qualifier from the second drop-down menu. Enter your criteria in the last field.
Select Add search filter to add another layer of criteria for a more precise search. Repeat as necessary.
Select Search to execute the search.
Select the radio button of the desired audience.
Save or Save/Publish
Select Save or Save/Publish at the top of the page.