There are two places you may Activate from. The first is the initial splash screen which appears when you run DBT.
The second is from the Help: About DBT... dialog.
When you click on either "Activate" button, the following dialog will appear. Select an appropriate radio button. Your choice essentially depends on whether or not you have an Internet connection.
The Activation Wizard will lead you through the process of activating DBT on your computer. It works most easily through an internet connection, but will offer you other ways to activate if you do not have, or cannot use, your internet connection.
The Activation Wizard may prompt you for software registration information (your contact information) along the way.