CryEninge
Download here:
http://www.crydev.net/dm_eds/download_detail.php?id=4
Getting started:
http://freesdk.crydev.net/display/SDKDOC2/Getting+Started
Also install:
DirectX
http://www.microsoft.com/en-us/download/details.aspx?id=8109
Extract then run DXSETUP.exe
Notes:
SANDBOX is the name of the normal editor which can be found under bin64/32