Every timer I try to load the XFrog Billboard trees (Poser pp2's with alpha maps), they come out invisible.

Mapping the texture and alpha  to a plane works fine, tho, but I would rather be able to import them directly using the Poser importer as that would be easier. Is there a solution to the invisibility problem?

Do they render ok , anything with a high transparency tends to disappear in the work window until rendered.

You can select a textured wireframe preview mode to make them stand out a bit for positioning

No, they render invisible.

I have the same problem when using one of the prebuild scenes, found it's a matter of size and placement. The prebuild scenes work with very large terrains 36k x 36k , your avg PP2 billboard is simple too small (1k x 1k) ) to be visible, plus it's imported at XYZ position 000 (the boondocks as far as the prebuilds are concerned). 

To have a visible and workable billboard, in the Assembly room select the Billoard in the Instances list, go to the Motion Tab , set the X and Y size to 2000 and change the XYZ position to 15000 5000 1750 and take it from there.

I don't have the problem when importing the billboard as a PZ3 (does require some extra work in Poser ofcourse) , when importing as PZ3 Transposer will allow you to set the Poser-Carrara scaling, which works fine at [80].