The VisitorRego software is downloadable, therefore the subscription key serves as your own personal key to unlock the software.
A subscription key is specific to each computer running VisitorRego. These are updated yearly or as required. These are built around the MAC address of the PC on which you are installing and can only be issued by VisitorRego.
These keys are distributed as two files,Subscription.txtandVROBJ.dll.
You need both files in the VisitorRego.NET folder to be able to launch Self Registration without the error you are experiencing.