Just a few notes on getting the VirtualBox Guest Additions to work on Ubuntu 14.04 alpha1.
1. Unknown version of X
"Installing the Window System drivers
Warning: unknown version of the X Window System installed. Not installing
X Window System drivers."
Explanation at the following links. You need to use a newer version of the Guest Additions.
virtualbox and the latest 14.04… display stuck at low resolution
2. Headers for the running kernel not installed.
The message is misleading. In my case it was actually missing dkms so before you install the addition run
sudo apt-get install build-essential linux-headers-`uname -r` dkms
Above helped with getting the Guest Additions running but still had some issue. Looks like graphics resolution is ok but Unity not running. I will investigate and update more.