You have to be root to be able to edit it. Also, you don't need 'execute' extentions in Linux/Unix. The OS detects what is executable without .exe, .bat, etc.
For single commands, put it in there, but for multiple commands, I create a script that is called. I had a friend at work who was having boot problems, and saw that he had added about 2K lines worth of code to his rc.local.