To request access to Audiences, fill out the form here. To use Audiences, visitor ID service and regional data collection has to be set up for your company. For information on how to set up Audiences, there is a PDF tutorial available here.
Once Audiences is enabled, it is accessible from the left side navigation. For more information on how to create Audiences check the documentation here: https://marketing.adobe.com/resources/help/en_US/mcloud/audience_library.html