Please make sure that your printWin(e) function is the same as the one in the KB article, at least its beginnig where the reference to the document object of the content page is obtained:
1) you need a reference to the RadWindow, as this button's context is actually the main page. This is one way to do it:
var oManager = GetRadWindowManager();
var oWnd = oManager.GetWindowByName("RadWindow1");
or you can use:
oWnd = $find(
2) you need to access its content frame (i.e. the iframe where the page's document object is located:
content = oWnd.GetContentFrame().contentWindow;
This is explained in greater detail in this help article
on accessing RadWindows.
3) the content object now holds a reference to the window object of the iframe, holding your page, thus you should be able to access the document object as well. This means that if all your references above are correct and the content page in the RadWIndow comes from the same domain this approach should work.
Please examine the above information, the KB article (and the project attached in it) and the linked help article and try to locate any major differences that may be resulting in your difficulty. If you are still unable to resolve this situation I would advise that you open a new support ticket and send us a simple project isolating your issue so we can investigate it further.
the Telerik team