Audiences that you would like to target with a campaign, must first be associated to the Advertiser level. To associate a Third-Party Audience or Onboarded Audience to your Advertiser, open the ‘Audience’ tab, find the Audience you would like to associate and select the red ‘Associate’ button. Then select the name of the Advertiser for which you would like to use this Audience from the dropdown.
To view associated Audiences, go to the Advertisers tab, click on the name of the Advertiser, and expand the ‘Audiences’ bar. These Audiences can then be associated to any Line Item created under the Advertiser. For more information on associating an Audience to a Line Item, see here: Associate Audience to a Line Item.
For more information on the metrics displayed for Audiences see here: Audience Metrics.