The unit called dsgnintf has been deprecated and is not normally on your path. It's name has been changed to Designintf.pas. Its property editor has been moved to a new unit called DesignEditors.pas. The constants from dsgnintf have been moved to DesignConsts.pas and the menus to DesignMenus.pas. Finally, the variants are now in Variants.pas. Instead of IFormDesigner use IDesigner. When creating components, add DesignIde to your requires list.
See Also : http://www.howtodothings.com/showarticle.asp?article=209