You can not directly get admission in MS in IIT kgp. MS in IIT kgp is purely project based.
You need to first appear for project interview of SRIC ( please see the link : http://www1.iitkgp.ac.in/topfiles/sric_job.php )
If you get selected in project , then after working for 6 months , you are eligible to appear for MS admission test and interview.
And one more thing , there are other interdisciplinary departments in IIT kgp , which offers MS in computer science subjects. But , my suggestion is that please do not go for them , as nobody will recognize later.
And , one more thing , apart from MS (CS) , other MS dept in IIT kgp , has almost no placement opportunity.
And if you compare MS (CS) and MTech (CS) , MTech (CS) has undoubtedly best placement there.
Source : my own experience.
Edit :
Please go through the following google group link in which the MS students of IIT kgp have mentioned how they are discriminated in terms of placement opportunities :
https://groups.google.com/forum/#!topic/rs-dept-rep-iit-kgp/IPR2N_Nbr7U
So , in a nutshell , MS <> MTech in IIT kgp at least.