There are two ways you can add existing content to your campaign.
In your content overview page
Directly in your Campaign
In your content overview page
Step 1. Select the content that you want to include in the campaign by clicking on the checkboxes next to them.
Step 2. Click on the 'Actions' button on the right-hand side and then click on 'Add to Campaigns'.
Step 3. Choose the campaign (or multiple) that you want to add the content to via the dropdown. Click on Update when ready.