Content of Media Kit is generated from attached files to post, if checkbox „Use attached files as media kit“ in post edit form is selected. Otherwise, you must create a set of media files in the Media Kit section in the backend of WordPress - see Media Kit in the left navigation.
Media kit can be added to Cart and downloaded as a single zip file.
Make sure your media file is attached, if not go to Media, find your file, and attach. Select the post you want.