If the operating system does not recognize the file type, then it don't have the app to work with this format. Select one of the programs commonly called "emulators", to be installed in your computer. The most logical is to use to work with mds/mdf files for the programs from the same company, which was engaged in its development. Emulators Alcohol 120% and Alcohol 52% can be downloaded from the manufacturer's website (http://alcohol-soft.com). However, they are not both free, while other companies and individual authors to distribute their own versions of emulators, without requiring anything in return. For example, the website of the company DT Soft you can download the free app Daemon Tools Lite (http://daemon-tools.cc/rus/products/dtLite).
After selecting and installing the program, run the file with the extension mds and mdf will be the same as any other - for example, by double clicking with the left mouse button. If you use Windows, an alternative way to mount a disk image highlight the file in the Explorer window and click Open button in its menu. Another option: click the right button, in context menu, expand the "Open with" and then select the name of the installed program. All they need to use these functions, the application information written in the system registry during the installation.
You can do without a file Manager of your operating system, only the window itself is installed on the emulator. To do this, run the program by using its shortcut on the desktop or item in the main menu of the OS. Then select from the menu to mount the disk image. For example, in the emulator, Daemon Tools Lite you need to either double click the row in the list of recently opened images, or you can click the icon "Add image". This will open a window where you need to find and select the. mdf file and then click Open. Everything else the program will make itself to the list of CD drive or DVD drive is added to another, simulating the disk from which you copied the image file.