Mac:
1.) Go to the download folder, extract the .zip and copy the file folder.
2.) Find your username, select "library", then "application support", then "minecraft", then "saves"
3.) Paste file folder into "saves" folder
Windows XP:
1.) Go to the download folder, extract the .zip and copy the file folder.
2.) Go to my computer. Go to Users. Go to your profile. Select "AppData".
3.) Now select "Roaming", and then ".minecraft", and open the "saves" folder.
4.) Paste the folder into the "saves" folder.
Windows 7:
1.) Go to the download folder, extract the .zip and copy the file folder.
2.) Hit the start button. In the search, type in "%appdata%
3.) Select "Roaming", then ".minecraft" in that
4.) Paste the folder into the "saves" folder.
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Another way of getting more areas to play in is seeds. Seeds are how the map generates(in a nutshell). You enter them in before you make a map. To find a good one, visit this site and browse for one you like "http://www.minecraftseeds.info/"