I already answered your other thread and for your convenience I attached the reply below:
I assume that you are using RadControls for ASP.NET, not the RadControls for ASP.NET AJAX.
I suggest to examine this online demo
which shows how to implement the desired scenario by using an ajax request when the RadGrid and the RadWindowManager are on the same page.
In case they are not, please examine this KB article
which explains referencing content page in a splitter pane from the parent page and vice versa.
Let me know if you need further assistance.
All the best,
the Telerik team