No need to do anything in website, you are already eligible for CSAB 2nd Round.

Below is the rule-

Those candidates who have not been allotted any seat at the end of Special Round-I, and NOT EXERCISING the option of NOT WILLING TO PARTICIPATE IN THE SPECIAL ROUND – II shall be considered for the Special Round-II. 

Dear student, as you have not got a seat in Csab first round, you need not do any fresh registration for the secomd round. The choices you have filled in the first round will be retained for the second round also. You will not be allowed again to fill the choices freshly. You can however opt out of the counselling process if you do not wish to go for that.