You can use the Page_Load event of the user control where the RadNotification is to check your database. If there are messages the user needs to see - Show() the notification from the code-behind: http://demos.telerik.com/aspnet-ajax/notification/examples/servershowwithnewtext/defaultcs.aspx
. If you only need to show a simple string the Show(newText) overload will make the job simple. Otherwise add the desired button and checkbox in the ContentTemplate of the control together with a label or literal where the message will be shown. You can check for the value of this checkbox in the Page_Load event again. If it is checked - store the user preference in your database so no further notifications will come up (so this piece of code should come before the check whether Show() needs to be called). After that you can also set the Visible property of the notification to false to effectively remove the control.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed