Hi,
You can reach it through the designer. In Telerik they are called ScreenTips. Or you can set them through code.
Here is a link to the Telerik RiboonBar section for adding Screentips
http://www.telerik.com/help/winforms/ribbonbarfundamentalsusingscreentips.html
Here is a static (shared) method that you can use to add a screentip to any rad item
Hope that helps
Richard