Open a web browser window on your host, go to http://www.turnkeylinux.org/fileserver, scroll down until you locate the DOWNLOAD BETA section, and click on the Installable Live CD image link (at the time of this writing, it was labeled as 138MB ISO, but maybe you'll see a different size):
Wait for the open/download file dialog to appear, and save the ISO file to your hard drive.
Once the file is downloaded, open VirtualBox, and click on the New button ...
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