When you start the program you'll be prompted to choose whether to run the program in Trial, Professional, or Lite mode via the License Type dialog:
The Trial mode allows you to use the product with the full Professional functionality for a free 30-day trial period. If you choose the Professional non-trial mode you'll be prompted to enter your Registration Details.
You can change the License Type at any time using the menu option Help -> Change License Type, or Help -> Upgrade.