Why Are Some Object Model Members Not Supported?
|SharePoint Designer Developer Reference|
The Microsoft Office SharePoint Designer object model is based on the Microsoft Internet Explorer interfaces, which are designed for code that runs in the Internet Explorer browser window—that is, in a run-time environment. However, the Office SharePoint Designer object model is designed for running code in Office SharePoint Designer, which you use to design Web pages—that is, in a design-time environment. Many Internet Explorer interface members do not apply to a design-time environment and cause an error to occur when attempting to use them in design-time code.
At the time of this writing, many of these object model members are visible in the Visual Basic object browser but are not available for use in Office SharePoint Designer code.