There are three ways to go:

Buy an unlock key online.

This is the preferred method. You have already downloaded the complete product so you don't need to do that again, you just need to get a key to allow the product to keep working past 15 days and also stop the trial warning screen from coming up. Bonus: your configuration settings will be saved so you won't have to set everything up again.

Here's the link:

Click here ( http://www.netnanny.com/products/downloads.html#nn5 ) for Net Nanny 5 unlock key information

and click on "Purchase a Key".

Buy the product online.
This will work out fine but you'll have to first uninstall the trial, then download and install the product again, then configure your settings again.

Buy a CD at the store.
This will also work out just fine however you'll have to first uninstall the trial, then load your CD and install it, then configure your settings again.