Since the Stages Sync app does not come from the app store, your computer may pop up with a dialogue saying it is unable to install the Stages Sync app because it is from an "unidentified developer". This does not indicate a problem with the app, however, a change to your security settings may be required to install an app from outside of the app store.

For details on how to allow the Sync app, see this Apple user guide article. 

Note: Only use Stages Sync download links supplied by Stages Cycling and located on the website to ensure you are installing an authorized version of the app.