ok... are you trying to install the official OTA t-mobile ICS update for the nexus S... cuz that will brick it... att nexus s' need the att specific update and that has not been released yet unfortunately so i would stay away from that for now...
I got tired of waiting for the official update so i just went ahead and rooted my nexus and installed cm9 which is ICS w/ the added features like face unlock, android beam and google wallet that the offical OTA update leaves out
i was afraid to root my nexus cuz of the possibility of bricking it but as long s you have the right files it will be fine (right files meaning files for nexus S and not for galaxy s2 or w/e)
but its really a easy process, it maybe took me 30 minutes to root and install cm9
oo and also the rooting process is a lil different using a mac or windows computer but here is a link to how I rooted mine
[GUIDE] Rooting AND Unrooting NS4G using a Mac - xda-developers
and a little hint..... INCASE SOMETHING WERE TO GO WRONG DURING THAT PROCESS, BEFORE YOU INSTALL THE FILES ON THE NEXUS, MAKE SURE WHEN YOUR IN RECOVERY MODE TO DO A "BACK UP" OF YOUR DEVICE SO YOU CSN RESTORE IT.
if you have any questions you can send me a private message or hit me up on google +, these links are in my forum profile