The files you are referring to are formats used be rippers such as DaemonTools or Alcohol 120% to circumvent copy-protection by generating metadata (the mds file). Generally, the mds file can be ignored, since it's just metadata.
You can convert the mdf file (which is the actual data information) using a tool called "mdf2iso". Open Terminal, type
Usage is simple: In Terminal, navigate to the directory where the mdf file is located (using the cd command) and run
sudo aptitude install mdf2iso
mdf2iso dsc_image.mdf disc_image.iso