Measuring proximity for the beacon is available on the iOS and Android versions of the RadBeacon app. This feature is not available on the OS X version.
Select your beacon from the list of discovered beacons. From the Settings or Detail view, under the Action menu, select Range to measure the proximity of the mobile device to a properly calibrated beacon.
If the beacon has not been calibrated, please do so before attempting to range for proximity. Please see instructions for calibration here.