Installing an App you bought on JustFly is easy. Just unpack the content of the zipfile you received onto your transmitters SD-Card. If you received a (personal) license, it has been sent in a separate email. Just unpack that one as well.
After doing so, your SD-Card should look like this:
When browsing in the Taranis, you should find the same directories:
Installation done, ready for activation.
have fun and