var power = 0.0;

var forceToAdd = 5.0;

function Update ()
{
if(Input.GetButton("Jump"))
   {
   power += forceToAdd * Time.deltaTine;
   }

   if(Input.GetButtonUp("jump"))
   {
   rigidbody.AddRelativeForce(Vector3.forward * power);
   }
}

Simple, adds to the power variable when the Jump button is down. When it is released we add force to the rigidbody.

Let me know if there are any errors :)

no errors, worked well :) , thx a lot

oh well there is small error

there is: power += forceToAdd * Time.deltaTine;

should be: power += forceToAdd * Time.deltaTime;

Time not Tine.