Thanks for posting. It is because you have PHP version 5.5+ installed. The mysql extension has been deprecated in this version.
We have created a patch to counter this issue. Simply download and extract the patch file(s) and replace with your current Pie-Register folder.
Please note: Make sure you have created backup of your existence files so incase of any inconvenience you can roll back to what you had!
To download please follow this link: