The latest exercise system seems to have not been packaged correctly. When I try to extract the tar file using 7-zip, I get an "Unexpected end of data" error. The tar also lists only UX_718_VM_20160708.ovf and UX_718_VM_20160708-disk1.vmdk files. I don't find a .vbox file in there.
Thanks Sarada Satapathy for helping me out. For those who face the same problem, open VirtualBox, File -> Import Appliance and select the .ovf file in the tar. Follow on-screen instructions and you are good to go.