When you first set your page to use the DD Header, you will not be able to immediately drag & drop content in the header area. To activate the dd header drag & drop content area, you must first upload an image. It doesn’t matter what image you upload in the dd header. This is only needed to activate the dd header content area.
After you have upload your own header image, refresh your page by hitting shift + f5. You must refresh the page in order to get the drag & drop working. Once you have refresh your page, you will see the highlighted box around the header area when you drag & drop content in the header.