The type or namespace name 'DataGrid' does not exist in the namespace 'System.Windows.Controls' (are you missing an assembly reference?)

You can get through this error easily by following the steps given in the below link:

http://www.microsoft.com/downloads/details.aspx?FamilyID=084a1bb2-0078-4009-94ee-e659c6409db0&displaylang=en

Microsoft has released a patch for the system.control.data and system.control.data.design dll's

After you do this the errors will go off.

Still you may not be able to drag and drop the datagrid to the page.
To get rid do the below :
Right click the toolbox and select reset toolbox.


HTH


MJ