notifyIcon1.BalloonTipIcon = ToolTipIcon.Info;
notifyIcon1.BalloonTipTitle = "消息：";
notifyIcon1.BalloonTipText = "http://www.mzwu.com/有更新了！";
notifyIcon1.ShowBalloonTip(30);

//或：
//notifyIcon1.ShowBalloonTip(30, "消息：", "http://www.mzwu.com/有更新了！", ToolTipIcon.Info);