Instead of searching multiple threads which can be time consuming, here is one way to do it.
I'm assuming you want to work with a population...?
From Objects tab, (using rock as an example) select a population of rocks. Put some scale to it on the object scale tab of your new rock population - I used 10 to a 100 just for grins. Check the box at the bottom that says "Preview Instances", then hit the "Populate Now" button.
Wow, ton of rocks right, now let's paint them where you want them.
You have two entries for populations, the first entry is the properties we want. On the distribution tab you will see an entry area for "Use density shader", this is what you want. Select create a new shader/colour shader/painted shader. Once selected the checkbox "Use density shader" should now be checked, if it is not go ahead and check it.
Now hit the "Populate Now" button again and everything disappears, you are now ready to paint.
Go to the top of your preview window and hit the "Pause" button when the preview gives you a good view - otherwise every time you try to paint the preview keeps trying to render.
Next select the little icon on the same row as the "Pause" button that looks like a paint brush. Select start painting/Painted shader 01 - or whatever you named it. You will now have a cursor that will let you paint on the terrain. Every where you paint white is where your population will appear.
Hit the paintbrush button again and select stop painting when you are done and deselect the "Pause" button to see what you have.
I've attached a sample file of this for you. Enjoy!
JR
