Basically, you'll need to set up a parallel process which has inside it a conditional branch on timer set to run whenever Timer is XXX sec or less. So like:
Conditional Branch Time 0 min, 30 sec or less
Shake Screen
Branch END
That will shake the screen when the timer is 30 secs or less. I think the problem you encounter might be due to you having it set as Timer 0 sec or more, or you might not be putting the Game Over message INSIDE the conditional branch, not after it. Post back if you encounter problems.