I would use the
nano editor to edit the correct file for your shell, which is zsh, not bash.
Put this line in at the top of the file
typeset -U path PATH
Put this line towards the bottom and save:
Press the control key and the X key to exit (see the bottom of the nano window for more commands). If you then quit your terminal app, you can test for
mysql being in your path.
You can tell your shell with
echo $SHELL and if you want bash since all your instructions are in it, you can go back to that if needed with
chsh /bin/bash in which case you would nano the file ~/.bash_profile since each shell reads different start up files. I recommend learning
zsh and finding guides that work there, though.