It's probably because you're trying to drive an LED at around 24 to 30mA, from an output that can only drive up to 4mA! Change that 100Ω resistor to around 620Ω or even 1k (hard to say, for sure, 'cuz I don't know what kind of LED you are playing with, but, if it's one that came with an Arduino "kit", then it's probably the dim kind). Also, if that LED is the dim kind, and not of the Super Bright class, it may, at that low a current, be too dim to see. Maybe drive the LED with a transistor (Google it).
You gotta check out the datasheet ;D