Click the star at the top of the edge panel in which you want to put a folder. That will bring up a screen with available apps, and with the current setup of that panel on the right. Tap and hold an app icon in that current setup panel; a Create folder option will appear at the top, and you then drag the selected app icon all the way up to that folder at the top of the screen. This creates a folder with that app in it in the panel. You can then drag other apps into it. The key is you have to start with an app that's already in the panel and which you want to put in the new folder.