Installing the Guest Additions for VirtualBox on an Ubuntu 14.04 server guest is the same as for previous versions:
The Guest Additions install will need to build kernel modules so we need
sudo apt-get install -y dkms build-essential linux-headers-generic linux-headers-$(uname -r)
In the VMs "Devices" menu, select the "Insert Guest Additions CD Image..." or hit Host+D (Host is the right CTRL key by default)
Now that the CD is "inserted" into the VMs virtual optical drive, mount it:
sudo mount /dev/cdrom /media/cdrom
And finally run the install script:
sudo /media/cdrom/VBoxLinuxAdditions.run