To get it to actually record your heart rate (instead of just displaying it), you need to tell the watch you are working out. I do that by clicking the center button and pressing down twice to go to "run workout" but you can also pre-load workouts from the app onto your watch or click down once more for a gym workout. You then click the center button and it will start finding your heart rate. Once you do THAT, you have to click the center button again to actually start the workout (very important, I forgot to do that until halfway through this morning's run and I was SO MAD).
To stop the workout, you have to press the down button 6 times and it will say "end workout?" Click on the center button and it shows a triangle and a square. When it's on the triangle, your workout is paused; press the down button one more time to get the little bar to underline the square. Then click the center button.
The next time you open the adidas train and run workout on your phone, it should automatically sync your workout data. When you click on the three lines at the top left of the home screen, you can click on workout history. Your workout should show up on the workouts tab. If you click on it, you will see that white screen with the black letters that I posted above. When you rotate the screen (you might need to unlock your screen first if you have it set to not rotate), you can toggle between the heart rate and the pace analysis.
Hope this helps!!