Finally I've resolved my problem on my own, I had some big mess up with differents c++/cli types.
If that can help anyone having the same issue , here is my fix :
String^ mainCui = Autodesk::AutoCAD::ApplicationServices::Application::GetSystemVariable("MENUNAME")->ToString();
mainCui = mainCui + L".cuix";
Autodesk::AutoCAD::Customization::CustomizationSection^ cs = gcnew Autodesk::AutoCAD::Customization::CustomizationSection(mainCui);
Autodesk::AutoCAD::Customization::PartialCuiFileCollection^ pcfc = cs->PartialCuiFiles;