Directions will follow, but first the warning...
After typing the su command and entering the correct password every command you type is executed with permissions of the superuser until such point as you type the command "exit" or simply close the terminal window. That means you can do anything at the command line, up to and including erasing the entire boot drive or any other attached drive (If you type the command to do it). Point being, you must be very careful, and I don't want to be blamed if something goes wrong. You should always have a recent backup of your important files, whether you're tinkering or not.
The superuser (root) is disabled by default (for the aforementioned reason) , this is why you were unable to get the password correct previously. To enable it, open Directory Utility (in the utilities folder), then click the lock at the bottom of that window. Enter your account password. Then go to the edit menu and select "Enable root user". Enter a password to use as the root password.
You then will be able to use that password after typing "su" in the previous instructions.
I'll also note here, since you've likely never used pico, that after you make the edits to the .plist file, you press control-o to save the file (CONTROL, NOT the command key) and control-x to exit pico.
You might do well to find other instructions on the net for pico, though it is pretty simple.
It will ask you what to save the file as after you hit control-o, you should be able to name it whatever you want, as long as you end the name with ".plist".