You can position individual objects or whole populations by selecting it in the 3D preview (move your mouse over the center "x" of the population area until it turns yellow and click) then dragging on one of the directional arrows, or click and drag on the center point to "free move". You can also enter specific numerical positions directly into the Population or Object node settings which can be a more accurate way to work when you have specific positioning in mind, for example matching it with an imported terrain.
You cannot currently reposition instances *within* a population.
- Oshyan