An app is installed inside your account and is not on your domain until you say so.
In the Apps tab, under a running app, pick one of your domains from the list and press "Point here". The domain then serves that app, and its name appears as a link you can open.
To take it back, press the small x next to the domain name. The domain returns to ordinary web hosting and serves your files again.
Two things to know:
- The app has to be running first. A domain cannot point at an app that is stopped or failing to start.
- One domain serves one app. A domain already serving an app does not appear in the list until you unlink it.
If you have no domains on the account yet, add one first - the app has nowhere to go otherwise.