Instructions
Download the latest version of "DotA" from the official website (http://www.GetDotA.com). Save the "DotA" map file to your desktop or another location on your computer that is easy to remember. The file will always have a ".W3X" extension regardless of what version it is.
Locate the "DotA" map file you downloaded, right click on it and left click on the "Copy" option. Alternately, you can highlight the file and press the "CTRL" and "C" keys simultaneously.
Locate and open up your "Warcraft III" map "Downloaded" folder. The default location is "C:\Program Files\Warcraft III\Maps\Downloaded". If you installed "Warcraft III" to a custom directory, you will need to look in the folder you manually installed the game to find the maps folder.
In the downloaded maps folder, right click and press the "Paste" button. Alternately, you can just click an empty space in the folder and press the "CTRL" key and the "V" key simultaneously to paste the "DotA" map file. Once you have successfully pasted the "DotA" map you downloaded into the maps folder, you are ready to play. Start "Warcraft III" as you normally would and join a "DotA" game to begin playing.