Instagram requires that you connect your Instagram account. As of April 4, 2018, you can only display posts from an owned Instagram account, an account you have the login credentials for. You cannot display posts from unowned Instagram accounts, unless pulled in via a hashtag source.
For additional information about Connecting Social Media Accounts see our article.
Username: You can pull in the posts of an owned Instagram account that you have the login information for.
Hashtag: Pull in any public posts from an Instagram hashtag by clicking the "hashtag" tab on the new instagram source overlay and entering in the hashtag.
Location: If someone tags a location in their Instagram photo, you can pull all the posts from that location. If you know the instagram location_id then just add that, if not, click search, then search for the location. Click on the result in the list of results. If you don't see the location in the list, sometimes too many locations are too close together, and instagram only returns a small number of locations in a search, so you are better off entering the location_id.