I’m new too, but I find the way it works is:
- Click “Create” then “Upload from file” to load a design
- To add another design, or even the Premium features like text, clipart, or shapes, click the big “+” sign at the top of your screen where the first design is loaded. Select the second (or subsequent) file from your computer, or a premium feature if that is what your are adding. Then place it in relation to the first design.
The only caveat is, you can’t add a Catalog design to an existing layout. Only files from your computer or from the Premium options. You CAN, however, make the Catalog design the one you start with, and just open it from your dashboard, and then add files to it by using step 2 above.