1. A melee map is a map which complies with certain rules and is meant for normal play (like the games you get if you play ranked or coop but those use blizzard maps only). You can see if your map complies with the rules by checking its status which will be custom if it does not and melee if it does. An altered melee is where the actual races are altered in some way (eg allowing terrans to make science vessles) and is a form of custom game.
2. I used the "Data"/"Export Map Image...." tool. Obviously I made sure the render was set to mimic ingame view settings and at ultra graphics (sadly extreem is still locked until next patch).
3. It describes where the change must be done in your map status. If you do not know how to change it, look at normal melee maps like the ones that you get when playing coop (they are available for download in the editor).