The current release of PenPal Pro is version 2.2
This release is not compatible as-installed with the new Microsoft Windows 10.
But there is an easy fix, that will allow you to begin or continue using this app to track your correspondents and correspondence: download the "Microsoft Access 2013 Runtime".
It is available for free directly from Microsoft. Click here to go to the download page.
Choose the file with "x86" in the name and download it to your computer. Then run the file and follow the simple installation instructions.
The first time you launch PenPal Pro v2.2 you may need to select the Access app, and you may need to tell it to trust PenPal Pro if a warning appears.
(Why is this necessary? PenPal Pro v2.2 installs and uses the Access 2007 Runtime, which does not play nicely with Windows 10. Upgrading this piece to 2013 is free and easy, and lets the program work with Windows 10).