The simplest way to install the app is from your Ecwid Control Panel, Apps > App Market > Themes - and click on the Theme Designer app ...



If you are not logged in to your e-shop designer account, you will be prompted to log in or register a new account.
Now click the "Get the App" button:

You will be asked to log in to Ecwid, then allow the App to use your Ecwid Account.