In order to install the Playbooks for Outlook you'll need to open up the Office365 Store.
To do this click the Home Tab in Outlook then click Store.
Or if accessing it from the Office365 webapp do the following:
Once added you'll need to click "Notifications" or "Send & Track" and then you will be prompted to sign in with your Playbooks credentials.
What if I can't use add-ins?
Some organizations have add-ins locked. In this case you'll need to get permissions added to your Office365 profile to be able to access add-ins.