When you purchased PerfectIt, you should have received an email with your license key. If you don't have that email, you can get your details here. If that still doesn't work, please contact technical support.
To activate PerfectIt, either click 'Activate' in the purchase reminder dialog, or click 'Activate PerfectIt' in the ribbon. Then copy and paste your name, email address and license key from the email and click 'Submit'.