OnSceneGUIの入力例

Sample.cs

using UnityEngine;
using System.Collections;

public class Sample : MonoBehaviour 
{
	[SerializeField]
	private int count;
}

SampleEditor.cs

using UnityEngine;
using System.Collections;
using UnityEditor;

[CustomEditor(typeof(Sample))]
public class SampleEditor : Editor {

	private Vector2 buttonSize = new Vector2(60, 60);

	void OnSceneGUI()
	{
		var sceneCamera = SceneView.currentDrawingSceneView.camera;;

		var component = (Sample)target;
		var pos = sceneCamera.WorldToScreenPoint(component.transform.position);
		Handles.BeginGUI ();
		var countButtonRect = new Rect(
				pos.x  - buttonSize.x * 0.5f, 
				SceneView.currentDrawingSceneView.position.height - pos.y - buttonSize.y * 0.5f, 
				buttonSize.x, 
				buttonSize.y);

		var leftButton = countButtonRect;
		leftButton.position += new Vector2(-buttonSize.x, 0);

		var rightButtonRect = countButtonRect;
		rightButtonRect.position += new Vector2(buttonSize.x, 0);

		if( GUI.Button( countButtonRect, serializedObject.FindProperty("count").intValue.ToString()) )
		{
			serializedObject.FindProperty("count").intValue += 1;
			serializedObject.ApplyModifiedProperties();
		}
		if( GUI.Button(leftButton , "<-") )
		{
			component.transform.position += Vector3.left;
		}
		if( GUI.Button(rightButtonRect , "->") )
		{
			component.transform.position += Vector3.right;
		}

		Handles.EndGUI();
	}
}