How can I paint instanced objects onto another mesh in Blender?
its a bit long to explain, you can find old tutorials in youtube which still work.
Basically you create a group from the objects you want to paint example : tree ,rock ,flower…
then you create the surface you want to paint on. Example : terrain…
add a vertex group to surface object , and add a hair particle system. Goto particle render tab and choose “Group” and select the instance group. then in particle vertex groups tab, in density setting choose the surface vertex group.
now when in weight paint mode you can paint the surface and add instanced objects.
be sure you have enough subdivisions on the surface object and you also need to check advanced settings in the particle system ( for rotation of the instances).
Here is a already setup file you can examine > http://www.pasteall.org/blend/33439