On occasion, the Mac hard disk can become confused as to who has what permissions to do what and where.  There is a utility that can generally remedy this situation.  Proceed as follows:

• Open the Mac hard drive

• Navigate to "Applications/Utilities/Disk Utility"

• Double-click to launch the "Disk Utility" application

• Click on the "First Aid" tab

• Select your hard drive in the left panel then click on "Repair Disk Permissions"

Depending on the speed of your processor, the size of your hard drive, and the number of files that you have stored,  this process can take several minutes to complete.  Once this task is completed, attempt to install the software again. In virtually all cases, this repair does the trick.