The materials required for billboards are pretty simple: just create a Standard material with texture in the diffuse slot, and mask in the opacity slot. Next, create a "Forest Material", and put the Standard in the "Base Material" slot. You can use the materials included with Forest as sample (eg. from C:\Program Files (x86)\Itoo Software\Forest Pack Pro\matlib\standard.mat).
Once created the materials, put them in a library (.mat) and use the Library Browser to create each item. You can add them one by one, or use the importing tools for batch processing. Please, see the Creation Library
reference, we have rewritten this help section recently and now it should be easier to follow. If not, please feel free to ask us.
The procedure described above, is valid for all type of plants and libraries, since it uses standard textures and masks. Unfortunately XFrog includes only the raw textures/masks, not the billboard libraries or .mat files. Probably same for Key3D, although i don't know this product.
In any case, although the Library Browser is a great help, you can use the billboard materials directly on a Forest object. Just apply it to the "Material" slot in Geometry->Geometry list, once added a "Template" item type.