I originally tried installing SteamOS on Virtualbox 4.1.18. This wont work. Guest Additions on that version is not supported for SteamOS’s kernel. This guy (http://www.liberiangeek.net/2012/09/install-virtualbox-4-2-via-its-repository-in-ubuntu-12-04/) haz the links to update your repo, and I just went ahead and installed version 4.3.

After you have the latest version of Virtualbox. Download the steamos zip, dump the contents into a directory and create an ISO from it, using:

mkisofs -J -r -o ../steamOS.iso DIR

After the ISO was finished, I made a quick and dirty virtual machine configuring it for 64-bit Debian. Also make sure that the EFI support option is ticked in the system preferences, since the SteamOS/Debian installer uses UEFI. After booting and doing the basic installation, at least in my case, it will drop into an EFI shell. All you have to do is baby it to execute the proper efi profile:

FS0:\EFI\steamos\grubx64.efi

(or FS1: depending on whether you have the guest additions iso already mounted)

Make sure to boot into recovery, or switch tty buffers (if you didn’t) and login as root. I believe the root username does not have a password. Then install guest additions. Clicking on the guest additions button from the “devices” pane will mount the iso onto your virtual machines /dev/cdrom file. Therefore:

sudo mount /dev/cdrom /mnt

(sudo probably not required)

/mnt/VBoxLinuxAdditions.run

Restart you VM, boot the efi file, and login from gnome. Your final product should look like this…