http://www.ehow.com/how_2314896_fix-file-permissions-mac-os.html
How to Fix File Permissions in Mac OS X
Step 1: Open your computer and locate the file or folder whose permission you want to change. Right click (CTRL + mouse click) on the file and click "Get info."
Step 2: Click on the padlock icon on the lower right side of the box. This will prompt for your user-name and password. Fill in the required information. Hit OK.
Step 3: Locate user-name: Under "Sharing and Permissions" tab in the pop-up, locate the user-name whom you want to give access to your file. Click on the right side of the user-name, under "Privilege" and change to "Read and Write" if you want that user-name to have access to read and change the file; "Read Only" if you don't want to give them access to changing the file; and "Write Only" if you don't want anyone to see the contents of the file.
Step 4: Close the pop-up and repeat these steps on the other files that you want to change after you are done with the changes. Repeat until you are sure that only those whom you want to have access to your file have this privilege.