To use the instancing lab is pretty easy. You have to have Bryce 7 Pro. Bryce 7 Standard doesn't have the instancing lab feature.
First, you need a surface, such as a terrain, a plane, or an infinite plane in the scene. Then you need an object you want to multiply, such as a tree. After you have these in the scene. select the surface (such as the terrain, not the tree). With the terrain selected, click the letter 'E' that should appear near it. Now you're in the instancing lab.
In the instancing lab, you'll find all manner of tools to apply the object to the terrain. The first and most important thing to do is to make sure you have your intended object to multiply selected. So, in the top right, click on the drop-down and choose the tree (or your intended object to multiply). Then use the tools to apply the object to the terrain. When you are done, click the checkmark.
There are several tools in the instancing lab and you'll have to experiment with them to see what you can do. But the foregoing gives you the basics about the instancing lab. Have fun.