Executing sudo pcmanfm, after giving the warnings, should have opened a file manager window from which you can select then edit /boot/cmdline.txt....
As hippy says, the
sudo pcmanfm command is just so you launch the file manager as root, because you need to have root privileges to edit the
cmdline.txt file.
So the sequence is:-
in terminal:
sudo pcmanfm ...and wait for the file manager to load as root (this will have an orange warning triangle in the icon bar).
navigate to
/boot/cmdline.txt in file manager and then right click on it and open with
leafpad (the text editor)
Very carefully edit this file as described.
In a similar fashion (i.e. from file manager when running as root) navigate to
/etc/inittab and edit this file.
Replace KillerApp.bas with your Picaxe file (whatever you have written as a basic Picaxe program).
Have you built a suitable interface to invert serial lines as shown in the illustration?