Within Teamwork Desk, you can enable Teamwork via the product switcher.
Click the grid of dots on the right side of the main navigation bar and select Manage apps from the dropdown.

Note: Only site administrators have access to manage apps.
In the Manage Apps modal, click the green Enable button to the right of the Teamwork option.

Once enabled, you can easily navigate to Teamwork via the product switcher.

Note: Any existing agents on your Teamwork Desk site will automatically be added to Teamwork as collaborators. They will not automatically receive an email invitation to Teamwork.
For more information, see: Integrating with Teamwork