Harvey, I certainly agree that the population and object positioning controls need some work. Keep in mind of course that you're working with a pre-release Technology Preview so there is a ways to go. We will certainly be improving the object handling systems for the final release.
That being said I do think it's reasonably straightforward to work with objects right now. They key I would say is to not go too fast and jump in to a complex or unusual scene from the get-go. Start with the default scene and make a grass population. If you can get that to work (it should be dead simple), then add a procedural terrain and again adjust so that it works as expected. This simple approach should give you a better handle on how populations work and also give you more confidence that you can even make them work at all - there is nothing as singularly frustrating as never getting any results at all. Once you've been able to get those scenarious to work you can start getting more complicated, but don't necessarily jump straight to adding the grass to an existing scene. You may instead learn more by building up the example scene to a more complex level, keeping the grass population working as you go.
I've taken a look at your current scene and it's unfortunate that you couldn't include your original population since I can't now see what the problems might have been with it. I do note that your planet is heavily displaced and your current scen is significantly *below* the normal surface of the planet. This can cause problems sometimes. In particular it makes positioning of the population bounding box difficult since it is always referenced to the surface of the planet. The objects should still sit on the terrain properly because they are feeding off the Compute Terrain node which provides surface positioning, but the bounding box will appear to be in the wrong place, floating well above your terrain. The preview instances should show correctly at least.
With a few minutes of tinkering I've managed to get the grass rendering in your scene by scaling them up a bit and offsetting the base object vertically a bit. This is a quick fix when objects are below the terrain for some reason. Usually "Sit on terrain" will work but in some cases it doesn't seem to get proper samples for the terrain or perhaps is not accurate enough and thus you need to apply some manual adjustment to positioning, particularly in the vertical. This should be done through the base object that is being populated, inside the population node. You can see an example of this in the attached .tgd illustrating my changes to make the grass appear.
I hope this will be of some help. Do rest assured that things will get easier as development continues, and in the future I recommend you not spend hours struggling with a problem before asking for help. Don't assume the problem is you when you're dealing with early software such as this - it's probably the software's fault.
- Oshyan