I think it's important to note a few minor details real fast here.
There are at least 2 "credits". 1 being the money you start with when you press new game. 2 being the money you have in the campaign mode.
If you are trying to boost the starting credits, that's one thing. If you have a save file and you want to change the amount of credits you have saved, that is something else entirely.
To change your SAVED CREDITS, you need to create a new game (assuming you haven't), and then save it. When you save it, note how many credits you have (EXACTLY). Save and exit. Open notepad and find your "saves" folder in the starfarer directory. Open the XML that matches your character name, and press CTRL-F and type in the exact number of credits you had. Scan through until you find this number. Add a couple extra zeros, and save the file. Exit and load up starfarer. You should be good to go. Once you have done it successfully, you can go back and change the number to anything you like, whenever you like.