Ahem, Herb, i found it out! First off, if you haven't tried to extract the files, then do that...but if you have, then mabey you don't have winrar? And if you did both, then mabey it only works for windows? I have downloaded it AND extracted it with no flaws at all! It might just be your Computer...?
I don't use winrar, I'm on Linux. I doubt it's anything to do with my computer - I've installed dozens of other maps.
Ahem, ATTENTION! I have made my Super Mario World demo! It's in it's early stages, so expect some mishaps along the way! If you (somehow) get stuck in a pit, press the / button, and type" kill @p", or something like that! I recomend instaling forge, just in case!