how to use Obb file ?

I'm use "split application binary" to create .apk and .obb files. I upload .obb file to my server and download it to sd card successfully.

I have already download .obb file to sd card,
obb file path: /mnt/sdcard/Android/obb/com.harry.ttbcd/main.20140423.com.harry.ttbcd.obb

After download and write the data to sd card complete, I restart the app and use fellow code to read .obb file, but all code couldn't work well.

/// <summary>
/// Loads the start level.
/// </summary>
/// <returns>
/// The start level.
/// </returns>
IEnumerator loadLevel(string levelName){
print ("loadStartLevel1 start");
string uri = "jar:file:///mnt/sdcard/Android/obb/com.harry.ttbcd/main.20140423.com.harry.ttbcd.obb" +
"!/" + levelName;

WWW www = WWW.LoadFromCacheOrDownload(uri,0);

yield return www;

if(www.error == null){
Application.LoadLevelAsync(levelName);
}
else{
print ("loadStartLevel err " + www.error.ToString() );
loadLevelFromObb(levelName);
}
}

/// <summary>
/// Loads the level.
/// </summary>
/// <returns>
/// The level.
/// </returns>
/// <param name='levelIndex'>
/// Level index.
/// </param>
IEnumerator loadLevel(int levelIndex){
print ("loadStartLevel2 start");
string uri = "jar:file:///mnt/sdcard/Android/obb/com.harry.ttbcd/main.20140423.com.harry.ttbcd.obb" +
"!/level" + levelIndex ;

WWW www = WWW.LoadFromCacheOrDownload(uri,0);

yield return www;

if(www.error == null){
print ("start load level2 " );
Application.LoadLevelAsync(levelIndex);
}
else{
StartCoroutine( loadLevel("splash_scene_01") );
}
}

/// <summary>
/// Loads the level from obb.
/// </summary>
/// <returns>
/// The level from obb.
/// </returns>
/// <param name='levelName'>
/// Level name.
/// </param>
IEnumerator loadLevelFromObb(string levelName){
print ("loadLevelFromObb");
string filePath = "file:///mnt/sdcard/Android/obb/com.harry.ttbcd/main.20140423.com.harry.ttbcd.obb";
WWW www = WWW.LoadFromCacheOrDownload(filePath , 0);
// Wait for download to complete
yield return www;
if(www.error == null){
Application.LoadLevel(levelName);
}
else{
print ("loadLevelFromObb err " + www.error.ToString() );
}
}

I have add all scenes to Build Settings, but when I read obb file and load scene:
errors:
04-24 11:48:14.600: E/Unity(29889): Level 'splash_scene_01' (1) couldn't be loaded because it has not been added to the build settings.
04-24 11:48:14.600: E/Unity(29889): To add a level to the build settings use the menu File->Build Settings...


how to use .OBB file ? I don't want to use GooglePlayDownloader, I want custom download and use OBB file.

 

I want my package size more small