Under the heading Storage it'll say [Optical Drive] Empty. Click on that, and choose the .iso file that you downloaded from Ubuntu. Then start the machine. Go through all the prompts. The first password you choose will be the main one to execute commands at the root level, so make it strong. I chose yes to encrypt the home directory, used the entire hard disk for partitioning (others partition and independently encrypt as they see fit), left http proxy blank, chose yes to install security updates automatically, selected the LAMP server and OpenSSH server (standard system utilities already checked), left MySQL root password blank (you'll set it later), and chose yes on the question about the grub bootloader. When all is said and done, you should have Ubuntu up and running on your VM.