Crysis 2 SDK (sanbox 3) should be released by the beggining of the summer, with this SDK you will be able to mod CRYSIS 2 but not make games for cryengine
the Cryengine 3 SDK will be released later this year, with this one you should be able to make your own games from scratch
it wasn't released with the game because
1- it wasnt complete, the one that was leaked was very buggy
2- EA wants to get money out of the Map pack DLC which will be released next week