When you start an unregistered copy of PerSense, you are offered an opportunity to Register.

You can register PerSense by sending your registration code to josh@mathforum.org.
A numeric key to fully unlock the program will be returned to you promptly.
There is no charge for registration.

You can also pull down the Help menu and choose Register Per%Sense. You will be given a registration code, unique to your computer. Click continue once you have your registration code.