Where exactly have you pasted the RadEditor's declaration in RadGrid? Note that the GridColumnCollection collection could contains only columns but not objects of type Telerik.Web.UI.RadEditor?
My recommendation is to examine the code of the following demo, which shows how to load RadEditor in an EditItemTemplate
of RadGrid: Datagrid Edit Template
If you experience any other problems, please open a support ticket and send a simple fully working project that demonstrates them.
All the best,
the Telerik team