Go to Datto Downloads Page
Download the Endpoint Backup for PC's
Run installer on the computer you are trying to add to your portal
Once the installer finishes you will be promtped to Log into the Datto Partner Portal
it will auto navigate to:
Status → Endpoint Backup Status → Register Agent (top-right of the page).
In the Register Datto Endpoint Backup Agent window:
Once all fields are completed, click Register.
Finally, Confirm that the process worked and you can see the new agent in the Datto portal
Leave the Endpoint on untill it finishes it first backup