Do you want to add another complete page or do you want to add a page to a multipage element?
Add another complete page
If it is a complete page you are referring to go to the Display menu and click Manage Content on the display you want to add another page to.
Choose a page you want to add from the available pages list on the left (this is a list of all pages that have been created), it will then appear in the box on the right, click the down arrow to expand it and you can set the display time. You can have as many pages as you want in the Display content box, if you have one in there already just add another using the same method. Click Save Display and refresh to push the pages to the display.
Add a page to a multipage element
The multipage element allows you to create movement within a section of a page design to make more of your screen real-estate.
If you want to add an additional page to multipage element go to Pages, click on the page you want to edit, select the multipage element, click the cog icon to configure. In the Multipage Widget Properties box you can either click the down arrow on one of the pages to expand it then click duplicate to copy it and set your display time and transitions or click the add page button to create a new page from scratch within the multipage.