I can't install the latest maintenance updates.
First, make sure you are logged in as an "admin" user when installing the update. You need admin-level privileges to save files in the Applications folder. You can see if you are an admin user by opening the System Preferences application and clicking on Accounts. An "Admin" designation will appear next to any user with Administrator privileges.
If you have logged in as an "admin" user, then it is likely that the Ownership & Permissions settings for the Applications folder on your computer have been damaged in such a way that the computer thinks you do not have permission to install the Reunion files, even when you should.
In this case, you'll want to try repairing permissions using the Disk Utility program (/Applications/Utilities/Disk Utility) on your computer. To do so, open the Disk Utility program and go to the First Aid section. Next, select your hard drive name on the left, and click the Repair Permissions button. This may take a while.
Then, try installing the update again.