Every empty slide will show you the full list of Content Blocks you can add. You'll find the button to add new ones, down on your screen.
When you see grayed out Content Blocks it means whether the element has already been added (like Headings) or the slide is already full.
Keep in mind that no other Content Blocks can be added when there's a Video element in the slide. This is because it will be reproduced in full screen when you're presenting.
In order to Delete Content Blocks, look for the trash Bin icon in the upper right corner of the element editing options, both in the Outline and Design sections. Be aware that this action can't be undone.
Just at the side of the trash Bin, you'll find the button to Duplicate the Content Block. This action can only be taken within the Outline.
To Edit Content Blocks just basically click on it, both the Outline and Design views will give you different options to edit the element.
Within the Design editor, you can manually drag and drop elements to rearrange. Hold the Shift key on your keyboard to move freely without the grid guidelines.