A simple way to do this would be to use an Elk SL1 strobe and connect it to Output 2. They are hard to miss, though not the most stylish thing to mount on the wall.
Otherwise, you could connect an LED to a voltage output, or to a relay controlled by an output. Depends on how much current your LED requires. Just make sure you have an appropriate resistor to limit the current.
For a rule, you can use:
Whenever Every 2 Seconds
AND Any Alarm Any Area is Active
Then Turn Out3 ON for 1 Sec