You should be able to double click the Form1.vb file and find it opens up in design mode. Also you can single left click the Form1.vb file and then you should an icon you can press in the tool strip area of the solution files window. If neither work then check to ensure that the definition of the form is the first class inside the vb file as Visual Studio 2005 looks at the base class of the first type defined to decide if a designer is appropriate for it.
Phil Wright
Free user controls for .NET Windows.Form
http://www.componentfactory.com