As far as I understand you can't find your population at all. If this is the case, do the following:
When you create a population and you want to know where it is, look for the "area center", that's where the population is placed on your planet. Zoom out in the preview window, don't hit "set camera". Then type in some values (you can try hundreds or even thousands) for the area center in the first and third box (not in the second one, because it determines the height where your population is).
Look if you notice a white square changing place. That's your population. Adjust the placement and the size of your population (area length a and b).
When you check "use density shader" you can apply a distribution shader, to make your instances appear where you want them to).
Don't forget to reselect your render camera under "select different view cameras" to get your initial view again.
If you DID find your population, but can't see any object in the rendering, maybe your object is too large. Play with the values and also with the object spacing. Check "preview instances" and hit "populate now". If everything is OK, you should see the proxies appear.
Another tip: for testing purposes the lowest render quality is enough, but for the final render always use the best quality.
Hope this helps.
