ARCamera not Working(I think)

Hi, 

I am working on a cloud recognition application using Unity plugins and exported it to android platform. the recognition is working well. yesterday I added a unity GUI objects into the file. I create a new C# script and attach it to the ARCamera. I don't know why, but after adding that GUI objects, when I run the application on my phone(Galaxy S3), the ARCamera is not working. I can assume it's the ARCamera because all the GUI buttons and boxes are drawn, but the the screen behind it is dark and the camera is not working. I can still press the button (the animation of button pressed work) so I assume it's the ARCamera. 

Here's the question:

1. How to properly create a Unity GUI? Should I attach it to the ARCamera or should I add OnGUI function on the CloudRecoEventHandler?

2. it's out of the context, but I cannot change the ImageTarget Scale. My Marker is not square, it more like an A4 size, but the scale of the marker in Unity editor is always square, XYZ = (1,1,1) whenever I tried to change only the X or Y, all three changed into the same number. As the result my marker is stretched on the editor.

Here's my GUI script if it's needed. thanks for the help.

using UnityEngine;
using System.Collections;

public class GUIHandler : MonoBehaviour {

	// Use this for initialization
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {
	
	}
	
	void OnGUI()
	{
		//Interaction Buttons
		GUI.Box (new Rect (Screen.width - 150,Screen.height/2,150,Screen.height/2), "Interact");
		
		if(GUI.Button(new Rect (Screen.width - 140,Screen.height/2+30,130,50), "Feed"))
		{
			
		}
		
		if(GUI.Button(new Rect (Screen.width - 140,Screen.height/2+90,130,50), "Call"))
		{
			
		}		
		
		//Player Status Interface
		GUI.Box (new Rect (0,0,Screen.width,50), "");
		if(PlayerPrefs.HasKey("loginName"))
		{
			GUI.Label(new Rect (10,0,200,50), "Welcome back, "+PlayerPrefs.GetString("loginName")+"!");
		}
		else
		{
			GUI.Label(new Rect (10,0,200,50), "Welcome, Guest!");
		}
	}
}