Unzip and place the A-4E-C Folder into C:\Users\YOURUSERNAME\Saved Games\DCS.openalpha\Mods\aircraft
You may need to create new "Mods" and "aircraft" folders. Simply create a new folder named "Mods" within DCS.openalpha, and create a new folder within it named "aircraft"
Place the A-4E-C Folder into "aircraft". You're done! No other modifications are required.