By default the Mac operating system only allows apps to be installed that are downloaded from the App store.

In order to install the GPS Log Book Sync App you will need to change this setting.  After you have installed the Sync App you can change the setting back if you wish.

The latest Mac Operating System uses "Gatekeeper" to limit the installation of applications from "unknown" developers.

Please see the Apple support page link or follow the steps below.  Once you have enabled the installation of software from "Anywhere" YOU WILL BE ABLE TO INSTALL THE SYNC APP:

https://support.apple.com/en-us/HT202491

Gatekeeper gives you more control over what you install. You can choose the safest option and only allow apps that come from the Mac App Store to open. There is also the option of only allowing apps that come from the Mac App Store and identified developers. Or you can choose to allow any apps to open, just like previous versions of OS X.

Gatekeeper options are found in Apple menu > System Preferences… > Security & Privacy > General tab under the header "Allow applications downloaded from:"