When the "Welcome to Tapcast!" screen appears, click "I UPDATE MY WEBSITE" and then select the SHOPIFY button under the "Integrations" section of the screen. When you click the "INSTALL TAPCAST APP" button, you will be redirected to Shopify, where you will be asked to install the Tapcast app. After Tapcast is successfully installed on your Shopify website, you will be redirected back to your Tapcast account.
Please note that although the Shopify app is unlisted, it will still install the Tapcast SDK correctly. If you would rather install the SDK manually, please follow the instructions in the FAQ titled "How do I install the Tapcast SDK manually on my Shopify site?"