Files with the extension .mrd have been created by the program Math Resource Studio.

To open a .mrd file, you need to have either the evaluation edition or regular/paid edition of Math Resource Studio installed. You can download & install the evaluation edition of the Math program from the Download page at http://www.schoolhousetech.com/Downloads/ (note the limitations of the evaluation edition on that page) . But if you have purchased the Math Resource Studio program, you can download it again from your Customer Care Account at https://www.schoolhousetech.com/Customers/MyAccount.aspx .

Thanks for asking!