To add a product to the wall, click the "Add Product" button from any of the four panels or type "a":
This will take you to the add product screen:
Here you may choose from any of your product types. Click on "Add this Product".
Wrapped Products: If you are adding a wrapped product (gallery wrapped canvas, print wraps, etc.), you will be asked to choose the type of edge:
You will then see the resize product screen with your default product for this type applied. Resize the product to the size you want and you are finished adding the new product to the wall!