If the Bitmap node is Black, it means that WW could not find the bitmap. Double-click the bitmap node to see it's settings in the material editor. Then click the bitmap file browser... if it's not pointing to the location of the file, then that is the problem.
Remember that the VTF files need to be extracted from VPK files onto your computer (WW Pro)... and that if you don't have WW Pro those VTF files need to be converted to TGA files.
These two docs can provide the information you need:
- Unpacking GCF and VPK Files (http://dev.wallworm.com/document/167/unpacking_gcf_and_vpk_files.html) -- Read Convert Bitmaps Section
- Material Library Generator (http://dev.wallworm.com/topic/35/material_library_generator.html) -- Read Consideration and Notes Section