First, you need to remove MacKeeper and request a refund from the company. MacKeeper is worthless garbage that is currently the subject of a class action lawsuit for fraudulent marketing. You don't want it on your computer, and you don't want to trust the company behind it with your credit card. Hopefully they'll give you a refund - it's not unheard of - but there are no guarantees.
Next, there are some good ad blockers in Apple's Safari Extensions Gallery, such as AdBlock. However, I would ask why you're looking for an ad blocker? If you are having ads pop up that should not be there - for example, they show up on sites that don't contain any advertising, such as this one - then you have a problem. Using an ad blocker in such a case would only cover up a symptom while leaving a potentially serious problem in place.
My suspicion is that MacKeeper isn't the only junk you have installed, and that you may have installed some kind of adware as well. Take a look at my Adware Removal Guide, which will help you find and remove any known adware that you have installed.
As for the use of the ad blocker, once you have made sure your machine is clean... I actually don't like ad blockers. Sites use advertising to help them provide services for free. It is precisely because people are using ad blockers that advertisers have to start getting so sneaky, and are using tricks like the unethical installation of adware. I personally don't use an ad blocker of any kind... if the ads on a site are too obnoxious, I take that as a fairly reliable sign that I probably shouldn't be visiting that site.
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