1) If I am logged in to more than one Google account (via gmail, etc) in the browser, the iframes do not appear, so cannot see Datastore Admin. Solution: log out of all but admin account, or open another browser.
2) When you added this to the admin Application Settings console, did you make the old built-in in app.yaml obsolete? In app.yaml, built-ins: datastore_admin is no longer available. It is still listed in places like:
3) How often do the entities in the datastore admin list get updated? For example, in a new app, it shows I just have one entity. However, in Datastore Viewer, there are several.