You can run the Ivy program "standalone", meaning, you don't have to import an obj model.  Simply double click and then click grow.  Click grow to stop (growing) and then click "birth".

This makes it a little easier to see your slider changes.  Double click again to start a new "ivy".  Export when you see something you like.

A little more information.  The material file points to three .png images for young and adult leaves and the branch textures.  You can edit the associated material file to use different images (textures).

When you import the obj into Bryce, you can ungroup it (make this easier by assigning a family color so you can select and group the obj easier.)  With it ungrouped, check the three textures to see if you need to set the blend transparency and maybe change diffuse or ambiance settings.