You've almost got it:

function OnGUI () {
    if (Input.GetMouseButton(1))
        GUI.Box (Rect (Input.mousePosition.x,Input.mousePosition.y,120,120), "");
}

But the box will be open only while you're holding the right button down - and it will follow the mouse pointer if it moves. If you want the box to appear and remain openned, do the following:

var boxOpen: boolean = false;
var boxPos: Vector3;

function Update () {
    if (Input.GetMouseButtonDown(1)){
        boxPos.x = Input.mousePosition.x;
        // Y runs up to down in GUI, so let's invert it
        boxPos.y = Screen.height - Input.mousePosition.y;
        boxOpen = true;
    }
}
    
function OnGUI () {
    if (boxOpen)
        GUI.Box (Rect (boxPos.x,boxPos.y,120,120), "");
}

You must set boxOpen to false to close it.

EDITED: Y runs upside down in GUI system, so it must be inverted (0 is Screen.height)

and to close with Left click i made this for others ^^

var boxOpen: boolean = false;
var boxPos: Vector3;

function Update () {
    if (Input.GetMouseButtonDown(1)){
        boxPos.x = Input.mousePosition.x;
        boxPos.y = Screen.height - Input.mousePosition.y;
        boxOpen = true;
    }
    if (Input.GetMouseButtonDown(0)){
        boxOpen = false;
    }
}

function OnGUI () {
    if (boxOpen)
        GUI.Box (Rect (boxPos.x,boxPos.y,140,180), "");
}