Can't paint trees or details

I'm working through a tutorial from the book "Unity 3.x Game Development Essentials." I'm at a section where I'm supposed to paint trees and details (grass) to the terrain, but when I try and actually paint, nothing appears. I'm using assets from the standard terrain assets package. I've tried Terrain->Refresh Tree and Detail Prototypes. I've also tried messing with the scaling of the models, both in the paint tools, and in the model assets. All to no avail. Is there something I'm missing?
EDIT: I did try zooming way in as well, as I have seen that to be a common problem for others.