First select and run the appropriate virtual box installation that suits your OS (or go to virtualbox.org for the latest version):

VirtualBox-4.3.16-95972-Linux_x86.run - for 32bit Linux - all versions
VirtualBox-4.3.16-95972-Linux_amd64.run - for 64bit Linux - all versions
VirtualBox-4.3.16-95972-OSX.dmg - for Mac OS - all versions
VirtualBox-4.3.16-95972-Win.exe - for Windows - all versions

Download the box.ovf and box-disk1.vmdk into the same directory. Then run the box.ovf file.

To run the VM image, use (for example) VirtualBox File->Import Appliance.
Choose file box.ovf.

The "openmodelica" user has password "openmodelica".

If the desktop is locked and you fail to login, you may have input the correct password but been instantly thrown out of the desktop.
Try the following work-around:
Push host-F1, host-F7 and input the password again
This resets the graphics drivers and makes xfce happy.

The host key is selected in VirtualBox. It is typically Right Ctrl, but OSX may use one of the Cmd keys (the button is shown in the bottom right of the VM).