cannot download fils from Dropbox

hi, i'm trying this code 

    public static String path= Environment.getExternalStorageDirectory().getAbsolutePath()+"/Bac/";
    public static String DropboxDownloadPathFrom = "BacMath/tst.txt";
    public static String DropboxDownloadPathTo = path+"dd";
//    public  static String dropboxdirpath="/storage/emulated/0/DropboxItems/";
    public static File Dir=new File(path);


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

        AndroidAuthSession session = buildSession();
        dropboxAPI = new DropboxAPI<AndroidAuthSession>(session);


        Dir.mkdir();
        Button b=(Button) findViewById(R.id.az);
        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                DownloadFromDropboxFromPath(DropboxDownloadPathTo,DropboxDownloadPathFrom);
            }
        });
    }

but no file is downloaded