On the virtual machine that you are trying to enable 3D support and finding the 3D support greyed out, if it is Windows 2008 R2, Shutdown the virtual machine, edit the virtual machine properties and go to the options tab. On the right there is a "Guest Operating System" box. Select Microsoft Windows 7 instead of Windows 2008 R2. Save it. Again edit the properties and select 3D on the Video card. Save it and start the VM.