To spawn object into the terrain, you first have to add some prefabs into the Library. If you don't know how to do it, please see this page first.
Select the prefab you want to spawn, then hover the mouse over the terrain and use left mouse to paint. Currently, there are 2 paint mode in this version: Spawn and Remove.
Spawn: this is the default mode, simply hold the left mouse to spawn objects. When you paint over the previously created instance, it's likely scale that instance up instead of spawning a new one. This mechanic gives the environment a more natural look since trees in the middle of a jungle often grow higher than the one on the edge. In some case, you want the spawned object to follow the surface normal vector, for example: rock on a mountain cliff, simply hold Shift and paint. In this mode, brush color will have cyan color.
Erase: Hold Ctrl and use left mouse to erase prefab instances. Note that it only erase instances of the currently selected prefab, and that instance must not be in any object group. In this mode, brush cursor will have red color.