After logging in, make sure you are on the Landing Pages list of the account.
Select one of the options on the right. You can create an AMP page as shown here: https://d.pr/qhZMTa, upload a .instapage file of a page that you previously downloaded, or create a standard page from a template or from scratch.
If you select Standard page you can start with a blank page or choose one of our premade conversion-optimized layouts. After selecting an option, the slideout for the new page will open so you can start customizing it.