The following steps explain how to prep the A2SERVER virtual machine, specifically VirtualBox, which has the notable virtue of being free. However, you can also use other virtual machine products, such as those from Parallels and VMware, and the process should be similar.
If you're comfortable doing so, you may also customize the steps below as you wish, or install Ubuntu Linux natively rather than in a VM.
Download Debian 8 amd64
Start VirtualBox, and go to Preferences.
Click Input. Set the host key to right-alt, and uncheck Auto Capture Keyboard.
Click New to create a new virtual machine.
Enter A2SERVER as the VM name, and select Linux/Ubuntu as the operating system.
Enter 256 MB for memory.
Under Virtual Hard Disk, leave the defaults (Start-up Disk, and Create new hard disk).
Choose VMDK for hard disk file type.
Leave the virtual disk file as Dynamically allocated.
Leave the virtual disk file location as A2SERVER, and set the disk size to 32 GB.
Click Create, and then again in the next window.
Click Storage. Click on "Empty" under "IDE Controller".
Click on the small CD icon on the far right of the window, select "Choose a virtual CD/DVD disk file...", and choose the Ubuntu Server file you downloaded.
Change "Attached To:" to "Bridged Adapter".
Change "Name:" to a wired network interface on your computer. If you don't have one, use a USB-to-Ethernet adapter. (A2SERVER won't work over Wi-Fi when running in a VM.)
Optional but recommended: Under Advanced, change the MAC Address to 080003F2FF59.
A2SERVER home page