Unfortunately we stopped supporting Boxcryptor Classic on new platforms like Windows 10, Mac OS X 10.11 "El Capitan", iOS9 etc.
I would suggest you to upgrade to Boxcryptor, because this version is supported on the new operating systems.
To decrypt your existing data, you should update Boxcryptor classic.
You will then have the possibility to decrypt and migrate your encrypted data.
The download can be found here:
https://www.boxcryptor.com/download/Boxcryptor_Installer.dmg (read only)
On Mac OS X 10.12 "Sierra" there are more Apple-restrictions present.
To mount Boxcryptor Classic tehre please use this version:
To decrypt your files on Windows 10 you can use Boxcryptor Classic Portable.
You can find the download here:
To migrate you files please follow these instructions:
I hope this helps your transition to Boxcryptor.