First have to say, that the Creation Tools part under Forest Pack is not so fully developed as RailClone which is much more suitable for tasks like that. Anyway there is workaround how to archive this with help of Spacing Tools (Shift + I).
This allows you to set Start Offset and Spacing needed. I'll recommend you to distribute with Spacing Tools newly created simple plane. Once these reference planes placed correctly, merge these objects into one by applying Edit Poly modifier and in Attach list select all of them. Now this object can be used by Forest Pack under Creation Tools - Reference Objects.
Hope this helps you.