Firstly, check the SPAM folder because some emails get sent to that folder due to various reasons. Most likely, the registration email is found in that folder, but if it isn't, use the Member Login page and log-in. This way you will have access to a number of functions exclusive to members.
This is whhat they say about the serial number: "If you have trouble to login, please Contact us to correct your serial number!"
This means you will have to wait for them to respond to your emails.
Member Area: http://www.perfectuninstaller.com/register/member.php