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The SplitPanel is not located in the VisualStudio toolbox. You need to drag and drop RadSplitContainer and add split panels in it. Please refer to the attached sample video (drag and drop over the browser to play).
Here you can find the documentation of this control for more information: http://www.telerik.com/help/winforms/splitcontainer-getting-started.html
Hope this will help you. Let me know if you have any other questions.