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 where you can collapse and open product categories from which to choose:
Here you may choose from any of your product types. Click on "Select":
You can move the product by clicking and dragging to a different place on the wall if you want.
If that's the product you want to add to the wall, click "Finish Add" and it will take you back to the wall with that product at its largest available size.
You can resize the product to the size you want by clicking the Resize Product button or typing "r" on your keyboard, or rotating 90 degrees by clicking the Rotate Product button or typing "o" on your keyboard.