Instructions
Navigate to the official MCEdit website (link in Resources) and click the button below "Download MCEdit now" next to the item that matches your operating system.
Click either the 32- or 64-bit package depending on what operating system version you have when selecting the Windows version.
Extract the contents of the .ZIP file to a separate folder on your computer once the download is complete. This folder does not have to be the same one as your "Minecraft" installation folder.
Run the "mcedit.exe" executable file to launch the map editor.
Click "Load World" and select the "Minecraft" map that you want to edit. The program automatically detects the folder with your "Minecraft" save games.