When creating a new Impress.ly site you will be asked whether you would like to include a Store in your site. Click or tap on "Yes, include a store" to add one to the newly created site.
If you've already created an Impress.ly site and did not include the Store during the new site creation process, you can add a store by opening your site, clicking on the "Manage Store" tab in the top horizontal menu and clicking on the blue "Add a Store" button.
On mobile devices, tap on the blue gear icon to go to your edit settings. At the bottom of the screen, tap on "Add Store".
In the next screen tap on the blue "Add a Store" button to confirm.
Once your Store has been added to your site for the first time, an overview of the basic requirements for the Store will be shown. This view is called the Dashboard and can be referred to at any times in the left vertical menu of the "Manage Store" tab.
You can find more about these steps here: