The RadTabWorkspace actually inherits from the old RadTabStrip control. The RadTabStrip has since been replaced with the RadPageView control and the team hid it from displaying in the toolbox in one of the past releases. Due to this, the RadTabWorkspace is inherently hidden from the toolbox as well. In order to get around this, you can modify the RadTabWorkspace.cs file in the TCEK to contain the ToolBoxItem(true) attribute.
RadTabWorkspace : RadTabStrip, IComposableWorkspace<Control, RadTabSmartPartInfo>
I hope this helps!