Need help with Animation Script

I want to play the walking and idle animation. However these are not being played. What did I do wrong?

function Update()
{
	if(Input.GetButtonDown("W"))
	{
	animation.Play("walking");
	}
	else
	{
	animation.Play("idle");
	}
}

It says that the Input Button W is not set up??

Thanks in Advance!

Its looking for W input in Input manager. Use Input.GetKey(KeyCode.W) to take input direct from Keypress (or GetKeyDown)