Ah, you probably missed it amidst all the shops and their signboards and you've probably walked much further down NSC Bose Road than you intended to - so now, turn around and walk east: there! The high wall behind the shops, you see it now, don't you? Where the wall ends is the doorway and now you can go in for your darshan of Chenna Malleswarar.
There is probably an easier way to get in - as both temples share a common tank, I guess one can skirt around the tank and get to one from the other. I have not tried that so far, though!