Having trouble downloading Arduino version 0021
First, what operating system are you doing this for? Second, why 0021 instead of 0022?
What programs will open the software as it says it's unsupported?
This question does not make sense. You are downloading a compressed file. Which application uncompresses the downloaded file depends on which operating system you are using. "It" is a pronoun. In your sentence it is used twice to refer to different nouns, but neither noun is defined. "It says"? What says anything? "It's unsupported"? What is unsupported?
Where can I download?
Lots and lots of places on the internet to download lots of stuff. If you could be a little more specific...