Hi, there is not an option to scatter in a volume directly, but it can be done linking Forest to other scatter object. In this way:
1) Create a small plane, and use Max Compound Objects->Scatter, to scatter them in the volume.
2) Turn on Display->Hide Distribution Object, to hide the volume source and get only the scattered planes.
3) Create a FP object, and assign the Max scatter as a UV Surface. Also turn on the Surface->Auto checkbox.
4) Adjust the Distribution settings to get a single item by plane.
That's all. Now you can modify the Max scatter, and Forest will be updated automatically.