I followed the guide to installing it, I have the folders where they're supposed to be in, but this is one thing I don't get.. I already have a resource.cfg by default. It was there when I installed the game.
Am I supposed to have it over write the other one?
I'm kinda scared of doing so, because it might crash, or not ever run ever again.

If a question like this has already been posted, I'm sorry, but I really have to know. :<

DON'T overwrite the resource.cfg file you already have (located in the C:\Program Files\Electronic Arts\The Sims 3\Game\Bin folder)! That one is in a different folder and must stay where it is or you'll bork your game. You need to place the downloaded resource.cfg in C:\Program Files\Electronic Arts\The Sims 3\ - not in any other folder.

I had the same thing happen to me so I copy the orginal resource.cfg file in the Files\Electronic Arts\The Sims3\ then replaced the file. It didn't bother the game. I always backup a file that I might be changing.