Userspace method: click "File" - "User Preferences" - "Add-ons" - "Install Add-on from File". The add-on zip file should contain io_scene_b3d directory, including the directory itself.
Alternative method: copy or symlink the io_scene_b3d directory to blender user directory, e.g. to %APPDATA%\Blender Foundation\Blender\2.79\scripts\addons\io_scene_b3d
Search and enable add-on in "User Preferences" - "Add-ons". Click "Save User Settings" afterwards.

Debugging

Userspace method: every time you make a change the script has to be reloaded using Reload Scripts command (F8).
Alternative method: my shortcut, Shift+Ctrl+D in Object Mode. It resets scene, reloads the script and imports test file.
Somewhat more alternative method (Windows only), my autohotkey script (see the autohotkey branch).

TODO

Import

Mind that animation is not yet implemented. Working on it!
Nodes use original quaternion rotation that affects user interface. Maybe convert them into euler angles.
Sometimes b3d files contain objects that were joined together in a single mesh (that's not my fault, see raw json data). I'm splitting objects with multiple meshes into separate objects (better than assigning different materials to individual faces) but I can't effectively split large meshes that use the same material. Maybe try proixmity-based clasterisation. Converting those meshes into a set of instances is even more complex, considering that differently scaled and rotated objects also can overlap.

Export

Exported files (script by Diego 'GaNDaLDF' Parisi) sometimes contain animation keys that go outside the animation. Assimp doesn't import them so I've added an extra frame, just to be safe. It's better to recalculate the animation using existing keys. UPDATE: could not reproduce, reverted. Will double check later.

License

This is all GPL 2.0. Pull requests welcome.

The import script is a heavily rewriten script from Glogow Poland Mariusz Szkaradek. I've had to rewrite all the chunk reader stuff and all the import stuff, because Blender API has heavily changed since then.

The export script uses portions (copied almost verbatim, just ported to Blender Import-Export format) from supertuxcart project by Diego 'GaNDaLDF' Parisi. Since it's all GPL-licensed, he shouldn't mind.

The b3d format documentation (b3dfile_specs.txt) doesn't have a clear license (I assume Public Domain) but it was hard to find, so I just put it here in the repository as well.

Alternatives

Assimp - doesn't read .b3d animation in most cases, maybe I have acquired a very particular set of files
fragMOTION - works fine most of the time, but it's a terrible nagware and the only suitable export is .smd

References

https://github.com/joric/gnome