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You can access images for Linux, Windows or Mac.

Just extract and import the appliance to your VirtualBox installation.

EDIT It would seem that there is “approval” given for reusing and rearming these images beyond the trial period – this screenshot is taken from one of the imported VMs in VirtualBox:

From Gordon Price on RFO:

Go to the Revit program folder (C:Program FilesAutodeskRevit Architecture 2013Program for RAC) and look for a file called AdskHardwareCertificationReport.xml. Rename this file AdskHardwareCertificationReport.xml.OOTB and accept the warning about changing file extensions. You will now be able to enable Hardware Acceleration just like you did in 2012. Performance will not be as good as direct graphics hardware, but it will be much faster than WARP.

Revit on the Mac (OS X) – Page 6

Read the whole thread if you want to understand some of the limitations and risks of this method.