Image via WikipediaFirst, you need to download a VMWare disk image (.vdmk). Here's a torrent file. Unpack the bz2 file to somewhere convenient. Next, open up Virtualbox (install), go to File > Virtual Media Manager and add the VDMK.
Either create a new appliance or add a second controller to an existing device. You'll need to change the network adapter to Intel Pro 1000 MT Desktop in order for the network to work.
Boot to the new hard drive and try ChromeOS out. There's not much to see, but it does launch fast, even in a VM.