How to make a point light which flashes when a key is pressed?

You can do this simply like this :

 var muzzleFlash : GameObject;
  
 function Update() {
  
     if (Input.GetKeyDown("D")) {
        //light flash code is here
        !linkedLight.enabled = linkedLight.enabled;
     }
 }

But what do you mean exactly by flash ? Or you can change the value : Light.intensity

==> Documentation here : http://docs.unity3d.com/Documentation/ScriptReference/Light.html

simply use the light.intensity

ie:

 var islighton = 0
 
 function Update() {
 
 if (islighton == 0 && Input.GetKeyDown ("D")) {
 
 light.intensity = 1;
 }
 else if (islighton == 1 && Input.GetKeyDown("D")) {
 
 light.intensity = 0;
 }
 }

You don't even need the MuzzleFlash variable, simply attach this to the light and it will work and you can also toggle between on and off :)

if you want it to just flash when key is pressed simply do this:

 function Update() {
 
 if (Input.GetKeyDown("D")) {
 light.intensity = 1;
 }
 
 else
 {
 light.intensity = 0;
 }
 }

i think it's something like that, i can't test this second code now, good luck :)

edit: you can use the first one for a flash light