I too am a linux n00b, but i believe all you need to do is the following form terminal or putty…
sudo nano rc.local
It is going to change directories to “etc”
Then the second command will open the rc.local file in nano (an editor). Using the “sudo” command will give you the proper rights to edit the file.
When you made your changes, press “ctrl+x” to exit. Then “y” to save changes. Lastly, just hit “enter” to save it as the same name (rc.local).
Anyone feel free to correct me if I am wrong.