I have no idea how to use the DateTimePicker control in WPF. It is not available in the Toolbox.
Please Note: The following answer only applied to WPF under the 3.5 Framework as NET 4.0 runtime has it's own datetime control.
By default WPF 3.5 does not come with a date time picker like winforms.
However a date picker has been added in the WPF tool kit produced by Microsoft which can be downloaded here. I guess it will become part of the framework in a future release.
It is simple to add a reference to the WPFToolkit.dll, see it in the tool box and distribute with your application by following the instructions on the website.
Before this was available other people had created 3rd party pickers (which you may prefer) or alternatively used the less ideal solution of using the winforms control in a WPF application.
Update: This so question is very similar this one which also has a link to a walk through for the datepicker along with other links.