your view should overide from CHtmlView,then call
CHtmlView::GetHtmlDocument
such as
MSHTML::IHTMLDocument2Ptr ptrDoc;
CSourceDialog *dlg;
ptrDoc = GetActiveIEView()->GetHtmlDocument();
dlg = new CSourceDialog;
dlg->Create(IDD_SOURCE);
dlg->ShowSource(ptrDoc);
dlg->ShowWindow(SW_SHOW);