It sounds like you are using a compiled widget from another dll as opposed to a simple widget (user control .ascx file).
If that is the case, you don't pass the path to the control, you pass an instance of the control itself. As an example:
var control =
var pageControl = pageManager.CreateControl<PageControl>(control, placeHolder);
I hope this is helpful, let me know if I have misunderstood your question.