not to make you sound dumb or anything, but is it actually turned on, have you toggled the on/off switch?
i have had 2 - 23" cinema displays since they came out.....i still, to this day, have to toggle the on/off switch to get it on. not because it is broken, but because i always think its on, but its off. im serious, it trips me up like every other day.
other then that, it should automatically recognize it, and just work. the only thing you should have to change, is the "display" options in "system preferences" where you can choose to mirror your laptop screen, or extend it. if you extend it, you need to choose where you want the external display to be in relation to your laptop screen. so you know which side of the laptop screen you know to drag stuff off of to get it to the external.
when i plug one of my cinema displays into my macbook pro, the screen on the macbook pro will go off, then turn blue, then everything comes back up. this happens every time i plug it up, and it happens with all macbook pros (at least all the ones i have used). that basically tells me that my macbook pro is recognizing the external cinema display.
sorry if it seems like we are repeating ourselves, or make things seem as if they are obvious. its just that it usually takes care of its self. the only other thing i can think of is it may be broken, or maybe your display port is broken?
i did forget to mention that on one of my macbook pros, i had to wiggle the mini display port connection to get it to work sometimes, but it always worked, it just had wiggly lines and messed up colors until i wiggled the cord.
