Iphone 错误：应用程序委托加载是未声明的标识符

我在iPhone注册页面编码方面遇到了意想不到的问题。 错误是

应用程序委托加载是未声明的标识符

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如何导入加载类？添加断点并检查首先出现错误的行。。。

   if (!isValid)
   {
      [AppDelegate showAlert:@"Alert!" withMessage:strMessage];
   }
   return isValid;
}

-(void)registerUser {
   [Loading startLoading:YES];

   NSString *urlString = [NSString stringWithFormat:@"url=%@",Access_Token];
   //firstname,lastname,email,username,password

   NSString *parameter = [NSString   
        stringWithFormat:@"firstname=%@&lastname=%@&email=%@&username=%@&password=%@",
        firstnameField.text,lastnameField.text,
        emailField.text,UsernameField.text,passwordField.text];

   NSConnection *conn = [[NSConnection alloc] initWithDelegate:self];
   [conn sendRequest:urlString withParaMeter:parameter withMethod:@"POST" withTag:1];
   [conn startAsynchronousRequest];
}