The SciLexer(64).dll has to be in the "search path" so that Visual Studio can find it at design time. Follow the instructions in the link below, paying particular attention to the section titled: Configuring the Search Path.
http://scintillanet.codeplex.com/wikipage?title=Installation&referringTitle=Documentation
Jacob
http://scintillanet.codeplex.com/wikipage?title=Installation&referringTitle=Documentation
Jacob