You need to specify the full name of the assembly.
(it's located in the GAC - If you have win 7 you can see the assemblies in: %windir%\assembly):
Type t = Type.GetType("System.Windows.Forms.MessageBox, System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
var method = t.GetMethod("Show", new Type[] { Type.GetType("System.String") });
method.Invoke(null, new object[] { "test" });