They are using the Crysis 2 MODSDSK, you are probably on the FreeSDK.
If you wish to mod Crysis 2, buy it, and use the MODSDK, if you wan't to create your own game from scratch and publish / sell it (without using Crysis assets), use the FreeSDK.
So you need a copy of Crysis 2 (Steam, Origins etc...) and the v1.1 of the Crysis 2 MODSDK :
dm_eds/download_detail.php?id=3