Hi,
I'm afraid, that at the moment, the Library Manager does not support handling multiple Forest objects. These objects need to be handled separately, as individual Forest objects. This is similar to the case of our Layered Lawns presets. The same applies to importing - although it is possible to import several items from the library simultaneously, these layered presets need to be added to the scene one by one. This is because a new Forest object is required for each of these presets.. We'll see what can be done in future versions.
Best regards,