To add content to your designs, you open them in the Editor. You can open multiple designs at the same time, as well as open templates and masters. Each design or Library item opens in its own tab in the center area of the window.
Opening a Page’s Active Design
To open the active design for a page, double-click it in the page tree.
Opening an Alternate Design
Each page in your project can have additional designs, besides the “active” design. When you select a page in the page tree, all of its designs are shown in the Page Designs panel below. You can open one of these designs by double-clicking its thumbnail.