If this is the Ivy Generator by Thomas Luft, then the output of the alpha maps for the leaves is in PNG?  Is Carrara handling the alpha on PNG maps correctly?  Indeed it could be screwed up UV coordinates, but the leaves are just mapped with a simple planar UV map.  There were cases of other packages not handling the PNG alpha channels for the leaves correctly, and users had to create their own alpha maps in another format for the leaves to show up in renders.  Keep in mind, the leaves this generator creates are simply single-sided planes.  So a double-sided material is best, if Carrara supports that, so they render on both sides.  Try creating your own alpha map based on the ones generated by the generator, and save them as a different file format, like just bmp or jpg even.  Doesn't matter if there's transparency information contained in the file, a simple black and white map of any kind can be used as opacity map in most standard 3d applications.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.