You can only add pages under the subfolders once the page is already created. To add under the subfolder you go to pages menu and under the folder you want to add the page, you click on the plus sign. Once you click, you need to chose the LINK icon option. Click on it and a box will pop, there you will add the name of the project and underneath you will see options of URL to add, click on content and chose the page you want to redirect to. Click save and is done.
Will show how to do it now.