Are you sure you DID download the Superuser-3.1.3-arm-signed.zip file?
Why not try (from the command prompt)
find . -name Superuser-3.1.3-arm-signed.zip
This will start in your current directory (which by having done the initial cd should be your HOME directory) and look for a file called Superuser-3.1.3-arm-signed.zip
If it doesn't find one, then it has NOT been downloaded