it's in the status bar at the top right of the screen to the left of the time, keyboard language, battery status etc.. it looks like a curved inverted triangle with grey lines if there's no network in range, black lines if there is a network and just a white empty inverted triangle if the card is off. click on the triangle and it'll give you a drop-down menu with the option of switching it on or off.
here's a picture of where mine is, from left to right:
adium, ichat, isync, airport, volume, battery charge, keyboard language, time, bluetooth and spotlight. you probably won't have all of those things but some of them should at least look familiar.