Ads on the Blog...or Not

Google says it will run ads on my blog page, for which I will be paid. This sounds great to me, but so far I have been unable to implement the change in the page's source code that would make the ads appear. Everything works fine right up until I'm supposed to paste the code in. Then nothing happens. I don't know why. But I'll be working on it. I have hope of success, if not success yet.