Sometimes installers for small system patches will impose OS version requirements, which means you will need to have at least the required version of OS X installed in order to apply the patch. While this usually is not a problem, some people may find installers claiming they do not have the proper version of OS X installed, when in fact they are fully updated with the proper version requirements.
When this happens there are a couple of things you can try:
Re-download the update
Try re-downloading the update from the Apple Support website, and apply it manually. Instead of Software Update downloading the installer package itself, this will provide it to you in a disk image which should help prevent the installer file from being corrupted.
Update in Safe Mode after performing general maintenance
Try running a General Maintenance routine on your system and then download the update manually and restart into Safe Mode. Apply the update when in Safe Mode, and hopefully the update will properly recognize the installed system software.
Reapply the latest "Combo" updater
Download the latest "Combo" system updater for your installed version of OS X (available at the Apple Support downloads site by searching for "Combo" or other similar keywords) and install it after performing general maintenance and booting into Safe Mode. This should refresh your system's installed components with those in the installer, and potentially clear up the problem.
If this does not work then a more in-depth option is to reinstall OS X, which is a relatively seamless operation in Snow Leopard. Boot to the install DVD and run the installer as you normally would, and the system software will be refreshed with new components off the DVD. Then be sure to apply the latest Combo updater before running any other software to ensure full compatibility, and finally try running the problematic software again.
After performing a full reinstall, most programs should work just fine but some may need to be reinstalled again. If you have a brand new Mac and are experiencing this issue when applying the latest updates at first launch, then we recommend you go ahead with a full OS reinstallation so you are certain the OS is installed correctly.
Install with Pacifist
One final option is to use the third-party package management utility "Pacifist" to install the update. This utility can be used to install individual components of installer files, or the whole package if desired. If you know your OS version matches the requirements for the software being installed, then this is another route to install the updated components and apply the appropriate permissions to them.