You need to have an element, text, or media item set as a foreground item.  If you click on the Media dropdown and add a media item as a foreground item, you will then be able to edit the Media attributes for that item from the Inspector.

You also need to make sure that the element you're trying to edit is selected. If you have an element on the slide, select it and then the inspector should highlight for you to change the attributes. If you want to change the attributes of the background, you need to click on Background in the Inspector.