Get and install virtualbox binaries from http://www.virtualbox.org/wiki/Downloads . Available for Linux, Mac and Windows, as well as many other exotic platforms. If your Linux distro has a package manager (such as yum or apt) try getting through there, since your distro might have made changes / patches to virtualbox.
Run virtualbox, click the add new button. In the appearing wizard, click next. Enter a random name (such as turnout web) and select the operating system to be linux / ubuntu. Click next.
Choose a base memory size for the virtual machine. Normally, virtualbox will choose the best for you, but you can tweak it if you think you can give it more memory. Click next.
Check the boot hard disk box, select use existing and then click the small button on the right. On the next dialog, click the add existing button and find Ubuntu.vmdk in the folder of your image. Don't worry about ubuntu-flat.vmdk. Click select and then click next. Click finish.
Now with turnout web selected, click the settings button and go to the network tab. Check that the "attached to" part is set to host only, not NAT or bridged. Save your settings.
Power up the machine. Wait for it to boot. Right before the login screen, you'll see a message box telling you the IP address for the machine. If the message box is empty (sometimes this happens because the network device hasn't started yet), press ctrl + alt+ backspace (right ctrl + backspace if you're on a linux box) to re-show the message box. Click OK, and wait for the login screen to come. Switch back to your host computer and start your browser. Enter the IP address you were given into the address bar, and you should see turnout. If you have any problems, post to the discussion board.