*sigh*  Alright, I'll explain this.
Once you download it to whatever your folder, be it your downloads or your desktop, it should be a .zip.  If you don't have winrar, than just right click and open it like a normal folder.  Inside should be another folder, that's the world save.  Now, there are two ways to transfer it to .minecraft, one is to open your minecraft, go to your resource packs, and click on where it says something like "open resource pack folder".  Once it's open, the folder you have now accessed is in your .minecraft so if you go back one it'll list all the assets.  Open your "saves" folder (there should be some other folders (this word is getting annoying) titled after your other worlds) and transfer the folder you took from the .zip file into there.  Next time you open minecraft and go to singleplayer, you should notice a new world, that's the one you just downloaded.  Enjoy.
If you've gotten this far, I'm glad you endured this.  If this tutorial hasn't worked, then just look up a "how to" on youtube, or something.

download the save file and place it on your desktop.

Next, either press "windows key+ R" and run "%appdata%", or go into your libraries and at the top where it has the file directory, type "%appdata%"

After you do this, locate your .minecraft file, and inside of that there will be a folder called "Saves"

Simply drag and drop the unzipped world save file into the "saves" folder and it will be under your singleplayer menu when you open up minecraft.

Hope this helps!