It seems on Ubuntu guest OS, there is some dependency issue when trying to install the VirtualBox Guest Additions by using the following packages:
sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11
The following errors will occur when the above command is run.
The following packages have unmet dependencies: virtualbox-guest-x11: Depends: xorg-video-abi-15 Depends: xserver-xorg-core (>= 2:126.96.36.1992)
The solution is to run the command below before you install the VirtualBox Guest Additions packages.
sudo apt-get install xserver-xorg-core
This works for me on XUbuntu 14.04. I am still trying to resolve the resolution problem on Windows 10 guest. It doesn’t seem the VirualBox Guest Additions have any affect on Windows 10 guest even it is installed without error.