You must adjust the density using the Distribution parameters, to get items within the painting area (the orange regions). In Forest the painting areas are parametric: you can modify the distribution, type of items and other parameters once painted.... basically you are painting a spline. In fact, the painting areas can be converted to splines and back. This method is much more flexible than inserting individual fixed items.
You can see examples of use in the Tree Lined Canal
and Modern Barn
In case you need more precision and place items individually, switch to "Custom Edit" mode in the "Tree Editor" rollout, and add the items from there.