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.
When you have received your license key, run PerfectIt and press PerfectIt's 'Start' button. When the purchase reminder loads, click on the 'Register' button. Then copy and paste your name, email and license key from the email and click 'Submit'.