On first start of Zoiper 5, you get a screen where you have a few options - Activate or Continue as a Free user.
If you click Activate, you will be presented with the Activation screen, where you can enter your Username and Password:
If you have clicked Continue as a Free user at any point and Zoiper 5 has been started, you can still activate, by going to Settings > and click on the Activate PRO:
When you click Activate PRO you will see the same Activation screen as above. Enter again Username and Password and activate Zoiper 5!