You are here Control Panel > Design Manager > Design Store.
Now that you are in the Design Store you will see a list of available themes. (See Image Below)
You are able to test the themes by moving your cursor over a theme in a list (See Image Below), Here you can see a description of the theme, a "See Demo" button and a "Use" button (See Image Below).
The "See Demo" button will take you to one of our demo sites for that template, this allows you to test a live example of the theme before downloading/purchasing.
The "Use" button will download the theme into your Design Manager and you will be able to customise and use it on your Website/Webshop.
Once you have downloaded your theme to your Website/Webshop you will recieve a confirmation popup (See Image Below) Here you have the option to go "Back to Overview" which will take you back to the theme list in the Design Manager or "Stay in the Design Store" which will close the popup confirmation and you will stay in the Design Store (See Image Below).