The latest releases of the P3 Control Panel utilize some Windows 10 exclusive features, so are no longer compatible with Windows 8 or 8.1.
If you are using Windows 8/8.1, the version immediately before we introduced the new Windows 10 features is 3.2.0.17, which you can download
here
It's unlikely that we will support Windows 8 in any future versions either. So until you update to Windows 10 you should stick with this version. If you're not limited to Windows 8 for work related reasons, it may still be possible to update to Windows 10 for free.