my computer says this update requires OS X version 10.8
Two questions .... what update for what product? What operating system version are you using today (it says OS X 10.8)? Click the apple on the left of the menu bar and 'about this mac' to see what operating system you're currently running.
Did you perhaps click the one that says 10.8.2 combo? Try selecting just the 10.8 and then update in your Apple dropdown <Softwae Update> for 8.2.
This assumes you are using a recent version of the OS that qulifies for upgrade as suggested above.
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