Input Axis Mouse ScrollWheel

I am loosely following a tutorial on some basic gun mechanics. Changing weapons is one of them.
Here is the error i receive

ArgumentException: Input Axis Mouse ScrollWheel > 0 is not setup.
To change the input settings use: Edit → Project Settings → Input
ChangeGun.Update () (at Assets/Scripts/ChangeGun.js:7)

When i go to Edit → Project Settings → Input I dont know what the middle mouse button is, or what i need to change about it.

Here is my script. Any thoughts?

#pragma strict

var weaponnum : int = 0;


function Update () {
	if(Input.GetAxis("Mouse ScrollWheel > 0") && weaponnum < 1 && Input.GetMouseButtonDown(1) == false)
		weaponnum++;
	if(Input.GetAxis("Mouse ScrollWheel > 0") && weaponnum > 0 && Input.GetMouseButtonDown(1) == false)
		weaponnum--;
}

Ah, you probably meant to have the “> 0” outside of the Input.GetAxis call like so:

function Update () {
     if(Input.GetAxis("Mouse ScrollWheel") > 0 && weaponnum < 1 && Input.GetMouseButtonDown(1) == false)
         weaponnum++;
     if(Input.GetAxis("Mouse ScrollWheel") > 0 && weaponnum > 0 && Input.GetMouseButtonDown(1) == false)
         weaponnum--;
}

Depending on the behavior you want, the second “> 0” might be better off as “< 0”