Where is this located in the toolbox? For the live of me I cannot find it right now. I even have a form that is using one of these controls and works correctly, but I still cannot find it to add one to another form.
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).