First, I'd recommend using CivNexus6 to create a .cn6 file rather than .nb2. This is now the default. Then use the .cn6 import add on ( io_import_cn6.py) for Blender 2.7+. The .nb2 format is the older Civ 5 format and is not as good in a few ways. Second, it looks like you are using the wrong script. You are using the older Blender 2.49 version of the .nb2 script with Blender 2.7+. You should be using the Blender 2.7 add on ( io_import_nb2.py) with Blender 2.7+ if you want to use the .nb2 format.