The DateTimePicker is in the Toolbox by default so if it's not there then you must have removed it. Note that it is a Windows Forms control so will only be available for Windows Forms Application or Windows Control Library projects. If you are creating one of those projects and you have removed that control then just right-click the Toolbox and select "Add/Remove Items".
I have checked at the add/remove item. The DateTimePicker checkbos was checked. But it does not appear in the toolbox. I unchecked and checked it again. But it still does not want to appear.
Any ideas? I haven't remove anything.