Unable instantiate android.gms.maps.MapFragment

I try to do a demo with google maps android v2 with very simple activity, just copy code from google page: https://developers.google.com/maps/documentation/android/start#adding_the_api_key_to_your_application

for activity:

package com.example.mapdemo;

import android.app.Activity;
import android.os.Bundle;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

for layout:

<?xml version="1.0" encoding="utf-8"?>
<fragment xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/map"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  class="com.google.android.gms.maps.MapFragment"/>

I had apply for a api key according to page and modify my androidmanifest.xml file, just like this:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.wenhai.driverschool"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="15" />

    <uses-permission android:name="android.permission.INTERNET" />

    <!-- add for map2 -->
    <permission
        android:name="com.example.mapdemo.permission.MAPS_RECEIVE"
        android:protectionLevel="signature" />

    <uses-permission android:name="com.example.mapdemo.permission.MAPS_RECEIVE" />
    <uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES" />
    <!-- External storage for caching. -->
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <!-- Maps API needs