There are several possibilities here. One could be that the applied force is too small another one could be that you apply the force to the wrong rigidBody or you don't even receive the keyboard input. Try the following:

function Update () {
if (Input.GetKey ("up"))
{
   rigidbody.AddForce (Vector3.foward * 10);
   Debug.Log("up pressed");
}
...
}

if you don't get a message in the console, you know that the keyboard never "arrives" correctly. If you get a message just increase 10 to 1000 or something way bigger. Trial and Error: just eliminate different possible error sources one after another.

To roll a ball, I recommend AddTorque (I.e. spin it) and let friction do the work. Otherwise you get unnatural effects, like in-air acceleration. For playability, a combination may be needed.

Oh, I see one of the problems.

You have spelt Forward incorrectly. It is Forward, not foward.

Also, there is no Backward in unity. You have to instead use forward but the negative of it.

For example:

if (Input.GetKey ("down"))
rigidbody.AddForce (Vector3.forward * -10;


//This will give the ball a force of -10 in the forward direction meaning the ball will roll backwards.

The problem is you forgot to put in the {}'s.

Wrong Way:

if (Input.GetKey ("down")) rigidbody.AddForce (Vector3.backward * 10);

...

Right Way:

if (Input.GetKey ("down")) { rigidbody.AddForce (Vector3.backward * 10); }

Never use the addForce and addTorque, basically anything that has to do with the physics on update. You must applythem on FixedUpdate