It is a free update, most users are getting it without a problem however some users seem to be having difficulty.
First thing the update is only for system running Mavericks. HA=ave you upgraded the OS yet?
If you have is Aperture installed in the systems Applications folder and named Aperture. These two points are important for Software Update to do its thing.
Have you tun Software Update and update from the Mac App Store. In my case it did not show up in the App Store until I had updated from 3.4.5 to 3.5.
Some users have had success getting the upgrade by uninstalling Aperture, reinstalling from their original media and then running Software Update. Remember if you try this you will need your serial numbers for Aperture so make sure you have them before trying it.