I'll presume that you have xterm installed and know something about using the command line...

'cd' to the directory where you downloaded the tarball and do this:

tar -xvvzf claws-mail_fix-date.tar.gz

This will extract it to the current directory.

In the tarball you will find

claws-mail_fix-date/
claws-mail_fix-date/README
claws-mail_fix-date/fix_date.sh

Then move (mv) fix_date.sh to wherever you want to store it.

Then configure the Action in Claws Mail, as specified on the FAQ page, but use the full path to the script in the Command Line:
e.g.

/home/user/fix_date.sh

HTH

ask for more info if you need it!

best regards

Paul