After the purchase you should have received the email with download link for Pro version’s zip. You can also download zip from your account page (https://wpfactory.com/my-account/downloads/). After you install this zip, you should see “Currency Switcher for WooCommerce Pro” plugin in your plugins list.
As for key (which is needed for automatic updates) – you need to install and activate free WPFactory Helper plugin (https://wpfactory.com/helper/), which will handle automatic updates, similar as it is with free plugins and themes hosted on wordpress.org. After you install it, you will be able to enter the key.
Hope that helps.