Most likely your computer does not know what an Alice world file is, with
extention .a2w. If the Alice world you download is called story.a2w, your
computer may think it is a .zip file and rename it to story.zip or
story.a2w.zip.
DO NOT unpack it or extract files, but instead rename it
back to story.a2w.
If you can't figure out how to rename it, you don't
have to. Instead when you are loading the file into Alice, you can load it
in as a .zip file if you select the option to load "ALL FILES", and not
just ".a2w files".
If you have trouble downloading an Alice world, sometimes I will put up
a .zip folder with the Alice world inside. A .zip file is a compressed
file that uses less space. If you download this, then you
will
need to unpack the .zip file. The Alice world cannot run if it is
compressed.
Then you can try to load the .a2w file.
It should have the correct name in this case.
Sometimes with .zip folders you can look at files inside them so you don't
realize that they have not been unpacked. But you do need to unzip the
folder
before using the Alice world.
Also remember that you cannot click on the .a2w file to get it to run. You
must start Alice, then File, open file and load the file into Alice.