To use the App, sign in to your e-shop designer account (or register for a new account) then click on the Ecwid App button, or go here to add the App to your Ecwid account. You will be asked to log in to your Ecwid account and to authorise the App.
Once Authorised, e-shop designer will be available from your Ecwid Control Panel: System Settings > Themes > Theme Designer.
The Theme Designer tab in Ecwid will list all of your Themes. Simply select the theme you want to apply to your store.
If you have a free Ecwid account, you cannot instantly apply a theme because that requires Ecwid API support. You will however, be able to get the CSS for a theme from the "Get Theme CSS" link and paste it into a Custom CSS Theme in the "CSS Themes" Tab of your Ecwid Control Panel.