The virtual hard disk controller the VirtualBox provides to the VM is probably currently of a type that is too new, so the virtual Ubuntu 14.04 won't recognize it. If you just added the virtual disk image into a VirtualBox VM configured for a modern version of Ubuntu, no wonder it did not work!

Typically, such virtual appliances come with recommended settings for the virtualization software. The kernel version 3.13.0-32-generic indicates this virtual appliance is based on a rather old version of Ubuntu: according to this answer on AskUbuntu.SE, this kernel version belonged to Ubuntu 14.04.

The message 'Unable to mount root fs on unknown-block(0,0)' means essentially 'I don't even know what kind of disk the root filesystem is supposed to be on.' If the end of message said something other than unknown-block(0,0), then it would mean 'I found the device that is supposed to contain the root filesystem, but I do not understand the contents, maybe there's something wrong with it?' This is generally what happens when a Linux kernel fails to find a root filesystem.

Ubuntu virtualbox kernel panic