1st. This is 3ds max glitch. Try restarting 3ds Max.
If You want to use scatter on multiple differently aligned areas, and default mode XY which scatters from top down does not fit, then You can use UV mode.
For that You need to do few steps:
1 Convert splines to editable poly. And use those surfaces (not spline areas).
2 Add UVW map on those ubjects (Forest Pack will use those UVs)
3. Most likely You will want to Adjust Surface durection to 0 (so objects dont orent to UP direction, but along the surface)
Hope this helps.
If You have any further questions let us know.