There's no app to download. Add Skybox to your home screen and it opens like one.

iPhone: open this page in Safari (if you came from an email, tap "Open in Safari" first). Tap the Share button (the square with the arrow), then "Add to Home Screen", then Add.

Android: open this page in Chrome. Tap the three-dot menu, then "Add to Home screen" or "Install app", then Add.

Then open Skybox from the new icon and sign in once with your email and password.