Problem Downloading Apps

Hello,

I have an older, all white, Macbook laptop from maybe 2008 or 2009 and its current version is 10.6.8 OS X.

I'm having problems downloading apps from the App Store on my Macbook. When I go to the App Store, I go

to the search bar on the top right and I type in, "cleanmydrive: external drives manager" because that's the app

that I want, and then when it shows up and I go to click "free" and then click "install app'' I get a pop up saying, "We could

not complete your purchase. CleanMyDrive can't be installed on Mac because Mac OS X version 10.7 or later is required.''

May I remind you, my mac is at version 10.6.8 and I tried clicking the apple symbol on the top left part of my screen and then

clicking "Software Update" but it says that my mac is up to date. So i'm not sure how to fix this. How can I further update my mac?

Can anyone help me with this?
It would be greatly appreciated!

Thank You!