This is typically due to damaged permissions on the hard disk drive. To alleviate this issue it is best to back up your saves and remove the game before repairing said permissions. The Sims 3 save files are located in:
User -> Documents -> Electronic Arts -> The Sims 3 -> Saves
They can be placed anywhere that easy for you to access and drop back into the folder after the reinstall.
To remove the game:
Delete “The Sims 3” folder from your Applications folder (or wherever you chose to install the game) also delete the “Electronic Arts” folder from your Documents folder and then empty the trash.
Once the game is completely removed navigate to the Disk Utility which can be found in Applications -> Utilities -> Disk Utility
Run the Disk Utility and select your hard drive in the left hand column and press “Repair Disk Permissions”
Once the repair completes you are good to install the game again and there should be no issues with the patch being installed.