Before an operating system can be installed, an ISO image must be downloaded from the Internet and attached to the virtual machine as a CD. The Ubuntu 14.04 LTS Server operating system can be downloaded from the following location:
Select the virtual machine in VirtualBox Manager and click on the Settings icon. Click on the Storage icon to manage the storage options:
Click on the first CD icon labeled Empty to modify the Primary IDE Slave controller. Click on the CD icon to attach the ISO to the virtual machine. When prompted, click on Choose Virtual Optical Disk File to find the ISO on the local machine:
Click on OK to close the settings window.
Repeat these steps as necessary to install the operating system on additional virtual machines.