<string name="settings_title">Sudoku settings</string>
이렇게 추가되어 있습니다~
근데 아무런 문제 없는거 같은데 안돌아가네요;;;;;

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="org.example.sudoku"
      android:versionCode="1"
      android:versionName="1.0">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".Sudoku"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".About"
         android:label="@string/about_title"
         android:theme="@android:style/Theme.Dialog">
        </activity>
        <activity android:name=".Settings"
         android:label="@string/settings_title">
        </activity>
 </application>
</manifest>
감사합니다...

모든 res에 올바르게 선언되어 있고
@Override
    public boolean onOptionsItemSelected(MenuItem item){
     switch(item.getItemId()){
     case R.id.setting_button:
      setting();
      return true;
     }
  return false;
    }

private void setting(){
     Intent i = new Intent(this, Settings.class);
     startActivity(i);
    }

이렇게 메뉴를 호출하고 있습니다...