On more recent versions of Mac OS X, you may be unable to install software such as programs you have downloaded from the Sussex website. Since Mac OS X 10.8 (Mountain Lion), the default option for Macs has been to install only software that has been approved by Apple.
If you receive a warning to say that the software can't be opened because it is from an unidentified developer, you will temporarily have to change a setting in the System Preferences to allow software to be installed from anywhere:
More information on this aspect of Mac software is given on the Apple website: OS X: About Gatekeeper.
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