Instructions
Launch "StarCraft II" and log in with your username and password.
Select "MultiPlayer" from the game options on the right and then click "Create a Game."
Type "DOTA" in the search box in the top right corner of the program window and then press "Enter." Click the DotA map you want to download and then click "Create Game" at the bottom of the screen. The map downloads to your computer; you can start a DotA game as soon as the map is finished downloading.