All you have to do is drag the save file from the zipped file into your saves folder in the .minecraft folder. If you can't find it, if you have a Windows 7, go to the Start menu, type %appdata% in the run bar, hit enter, and you will be in the Roaming folder. It is a hidden folder, so you can either do that, or you can go into your folder options and click show hidden files and folders. Then, you can go into your C:\Users\(username for the computer)\AppData\Roaming\.minecraft\saves. You can then drag the save file into the folder using an archiving program, like WinRAR, 7-zip, etc. Hope that helps.