First, you need to install the app from the Zendesk Marketplace. To do that, you must have admin-level permissions in your Zendesk instance.
After installing, the app is available on the app view when viewing and replying tickets. Please sign-in using your Transfluent account information, or make a new account. To enable the Zendesk app you must have a payment method registered for Transfluent as well as you need to enable the Zendesk service for you and other agents you which to have access to the app. Both are done in My Account, which you can access from the "Settings" tab in the Zendesk app or from here.
Once you have added payment method and enabled the users, you are good to go!