if I get it correct, you need to add namespace:
System::Windows::Forms at the beginning of MessageBox::Show().
Therefore:
System::Windows::Forms::MessageBox::Show();
should work well.
You can also use one of the following forms to call a function, use a
class, or use anything else in a namespace:
using namespace System::Windows::Forms; // allows your code to access
whole System::Windows::Forms namespace
using System::Windows::Forms::MessageBox; // allows your code to access
only System::Windows::Forms::MessageBox
If you, however, use latter one, you may not specify parameters for
Show method of the messagebox because parameters are also in
System::Windows::Forms.
Ismail