Please, try this:
- Reset Max and create a rectangle
- Create a Forest object usign the rectangle
- With the Forest object selected, go to the Modifier panel, open General rollout and click 'View Stats' button
- What is the 'Viewport items' value ? (this is the number of plants created)
First, we must know if the problem is that Forest is not creating items, or they are not visible in the Viewport. From here, we will investigate more in deep.