Access your wordpress site administration "wp-admin", then navigate to the plugins section and click "add new" In the search bar, you want to search for "Unlockr" You will see the Unlock Store Widget in the results, from there you can proceed with the installation.
You can also get the Unlock Store Widget from here directly : https://wordpress.org/plugins/unlockr-phone-unlock-widget/
Now that we have installed the widget from the wordpress store, the rest is very simple,
From the page where you want the widget to be added, you want to create a text block with the following line :
Make sure you replace the WIDGETSTOREID with the correct code of your widget. It can be found on the addons section of Unlockr when you click on "manage this addon" and go to the "preview & install" section.