When you log into your personal online dashboard you will see a list of the tests that you have purchased and are ready to use. If you haven't activated your online account so that you can access your personal dashboard, please see this article.
When you want to use your test kit, you will see an "Activate Kit" button to the right of the test name under 'Actions' in your account. Click this link to activate your kit and record your personal details and sample date and time. When ready, submit your details.