On the selected domain, navigate to the Admin panel by clicking on the Admin menu item 

This will open the Admin panel, click on Datasources

This will open the Datasources panel, where you can click on connect HubSpot

Once you click on connect, a new window will popup, asking you to login tou your HubSpot account. After you have logged in (or if you are already logged in), you can grant the access.

After that you will be taken back to Causalytics and now HubSpot is connected, and that's that.