Objects are, by default, laid across the top of your terrain when populated. I'd suggest you use the 'preview instances' button to get a feel for where the trees are placed after you hit 'populate now', then make adjustments to scale/distribution/etc. to fit your scene.
The default population will take its base building level from the compute terrain node being fed into the 'terrain shader' input of the node, however, if you have more displacements that appear after this(like in the 'shaders' tree) you will want to use the last shader that applies displacement to the ground to feed into the 'terrain shader' input.
The 'preview instances' checkbox will be the best place to start to get a hold on where each instance is placed.(If you have many, many thousands of instances, though, uncheck this box again before making any camera/node network movements etc. It will really slow the program down a lot when instances are being displayed).