In order to do The Index, you have to complete The Glast Gambit quest(a quest given by The Perrin Sequence(Need to complete The War Within quest). Once you completed The Glast Gambit, you can do The Index freely(excluding the credits needed to enter it). Each level has its own cost to enter and reward.
E.g., Low risk - need to pay 45,000 credits, you’ll get 75,000(plus what you spent) in return, but you have to get 25 points to get the credits.
Also, the longer the match goes on, the enemies become more difficult to kill, so put that into consideration.