When I try and authorise I get this message.The required directory was not found or has a permissions error. Correct this permissions problem and try again, or deauthorize this computer if the permissions cannot be changed. How do I rectify?

iTunes: Missing folder or incorrect permissions may prevent authorization

Mac OS X

1. Log in to your computer using an administrator account.
2. In the Finder, choose Go to Folder from the Go menu.
3. Type: "/Users" (without quotes) and click Go.

If the Shared folder exists

1. Open Terminal (found in /Applications/Utilities).Warning: This step involves modifying permission settings by entering commands in the Terminal application. Users unfamiliar with Terminal and UNIX-like environments should proceed with caution. The entry of incorrect commands may result in data loss or unusable system software. Improper alteration of permissions can result in reduced system security or exposure of private data. This option requires a non-blank admin password.
2. Depending on which version of Mac OS X you have, this step will vary:
   • On Mac OS X v10.5.8 and earlier, type:sudo chmod -R 777 /Users/Shared
   • On Mac OS X v10.6 or later, type:sudo chmod -R 1777 /Users/Shared
3. Press Return.
4. Quit Terminal.

If the Shared folder does not exist

The following steps will recreate the Shared folder if it is missing and ensure that it has been assigned using the correct permissions.

1. Open Terminal (found in /Applications/Utilities).Warning: This step involves modifying permission settings by entering commands in the Terminal application. Users unfamiliar with Terminal and UNIX-like environments should proceed with caution. The entry of incorrect commands may result in data loss or unusable system software. Improper alteration of permissions can result in reduced system security or exposure of private data. This option requires a non-blank admin password.
2. Type or copy and paste the following command into the Terminal window:sudo mkdir -p /Users/Shared/
3. Press Return.
4. Enter your administrator account password when prompted, then press Return.
5. Depending on which version of Mac OS X you have, this step will vary:
   • On Mac OS X v10.5.8 and earlier, type:sudo chmod 777 /Users/Shared
   • On Mac OS X v10.6 or later, type:sudo chmod 1777 /Users/Shared
6. Press Return.
7. Quit Terminal.