John,
Are you using the full namespace for the declaration (System.ComponentModel.Design.IDesignerHost)? I am able to do this without adding any references or using/import statements.
If using the full namespace doesn’t work (which I hope it does!) I did find examples of how to use this in both the SDK and some on-line articles. For the SDK check out the section on IDesignerHost Interface, there are examples in VB, C# and C++ here. A good on-line example, which illustrates how to add a control to a Windows Form from a VS add-in, is http://www.mztools.com/Articles/2006/MZ016.htm.
I hope this helps. If you are still stuck please send me a sample and I’ll see what I can do.
Thanks,
-Melody