Click the Descriptors Tab to go to the Descriptors Workspace.
Find the descriptor you are interested in, making sure you are in the right Descriptor Set. Then click the button in the Linked Media column. If there are no media files yet linked, the button will be red.
In the Linked Media pop up fine the media file you are interested in and click the Link button in that row.
The buttons in the Linked column will say what you can do for that descriptor.