This eluded me too. The solution is close the notebooks icon if it's open,then click once on it. You'll get a view of your notebooks that allows you to drag and drop them into stacks.
Stacks is not working for me. Macbook Pro / OSX 10.14.3 / Evernote Version 7.9 (457564 App Store) Editor: 62.3.7750 (ee6e44e). I have also tried on my iPad - current ios, current evernote, not working there. It's particularly curious because I have previously created stacks. Is this a bug from a newer version of Evernote?