The first thing to do is to re-register the application if you already have the license key that you paid for. I've also checked the official website of the developers, but I was greeted with a message that says the following:
"Contact Plus Corporation has shutdown. Due to business failure we no longer offer sales nor support of any existing software. Inquiries should be sent to webmaster at contactplus dot com."
Reinstall the application, then use the File > Open and point the window to path where the inventory was previously stored.