virtualbox.org

I did install fresh Win 10 enterprise and upgraded my existing Win 8.1.
After fresh install, I installed Toad for sql server, and opened the app with success.
Installed from guest additions cd, then Toad for sql server app failed to start.
There are no errors or warnings, but the pointer just shows that the app is starting, but nothing happens.
This same issue also arise after the upgrade from Win 8.1

Version 5.0.1r101573 Vbox
Guest additions V5.0.0

Since you are running a test build of VirtualBox you could try the latest versions for OSX.
https://www.virtualbox.org/wiki/Testbuilds
You should use the latest guests additions and extension pack as well.