Hi Ignacio,
The Forest Pack plug-in uses the item's Pivot Point position - most likely, it isn't "centered" in your case. To fix this: select the inner Forest object, go to the Hierarchy tab, and under Adjust Pivot, select Affect Pivot Only. Move the Pivot Point to the desired position (using the Align tool - Ctrl+A can be very handy to do this automatically).
I hope that helps.
Best regards,