If you are Apple Programmer or Developer then you must aware of this because it make very difficult work on windows with .dmg extenstions as compare to Mac OS X,Officially these kind a file are unsupported on Windows,This Guide is quite useful to know what dmg file is and how to open it.
Open a .dmg file and exctract to any location which you want
7-Zip is a powerful compression tool that is distributed freely to users. Its installation file is completely free If you do not have 7-zip installed on your computer, you can Download and install it on your machine.
Select a .dmg file and extract to any location which you want ,The program will begin extraction which will go as fast as your computer can manage