Hi,
Forest Pack doesn't have the ability to generate a collision mesh I'm afraid, except for the built-in display modes.
One option though might be to use Max's built in Substitute modifier.
To use it, first you'd need to create a collision mesh. (Here I just created a convex hull, but you'd probably be better off modelling something manually to better match the trunk)

Next, add a Substitute modifier to the object that you're scattering, and pick the convex hull from the scene.

Turn off In Render. In Forest Pack, set the display mode to Mesh and you should now be able to display different geometry in the viewports than in the render.

I hope that helps,
Paul