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The Garmin Vivosmart activity tracker application requires Mac OS X 10.8 or later version. If you are using Mac OS X 10.7 or below version, then you have to upgrade to latest version. You can download and install Yosemite from App Store.
Open App Store and click Mac OS X Yosemite in the right or search Yosemite. Then install it. If you are using 10.8 or 10.9, then it is not necessary to upgrade to Yosemite.
You can download Garmin application from here --> https://connect.garmin.com/en-US/start/vivosmart/express
If you have any questions or would like any additional information, please don't hesitate to ask.
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