What mode are you in? In the green square mode it is set to auto flash, in any of the programme modes (P, Av, Tv, M) yolu will need to actually tell the camera you want to use flash.
Your cameras flash pops up to fire af assist strobes in low light, you can disable this via the custom function menu, it will do this in any mode when AF is selected on the lens. If you want it fire the flash as well you need to press the wee flash button on the side of the pentaprism.
