To make a light flicker, you need a light entity. In the light's properties, go down to Appearance, and choose the light style that you want. You said that you wanted the light style to be like a fast flicker, like an alarm, but I've found that the slow, strong strobe looks better IMO for alarm lights, especially if they're red.
The tutorial "In the Beginning Part 4" covers lighting. Another example of different lighting effects would be my Disco Party level. Lol! Well, I'm serious. There are many kinds of lights throughout the level, and I've included the .rmf as well.