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After the installation package has been downloaded, double click to start the installation. By default your Mac will only allow installation from Mac sources and developers, so you will have to override the permissions as shown in this image. Click on Open.
In some cases, you will have to manually adjust the settings in your System Preferences to allow the installation to proceed. Under Security and Privacy locate the Allow apps downloaded from: and select Open Anyway when you see the Pinogy - POS installation package listed in the options, as shown on the screen.
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