object.Add (folderName) 

function AddNewFolder(path,folderName)
{
   var fso, f, fc, nf;
   fso = new ActiveXObject("Scripting.FileSystemObject");
   f = fso.GetFolder(path);
   fc = f.SubFolders;
   if (folderName != "" )
      nf = fc.Add(folderName);
   else
      nf = fc.Add("New Folder");
}

Sub AddNewFolder(path, folderName)
   Dim fso, f, fc, nf
   Set fso = CreateObject("Scripting.FileSystemObject")
   Set f = fso.GetFolder(path)
   Set fc = f.SubFolders
   If folderName <> "" Then
      Set nf = fc.Add(folderName)
   Else
      Set nf = fc.Add("New Folder")
   End If
End Sub