Imports and exports Three.js' ASCII JSON format.
Assumes Blender version 2.60.
Copy the io_mesh_threejs folder to the scripts/addons folder. If it doesn't exist, create it. The full path is OS-dependent (see below).
Once that is done, you need to activate the plugin. Open Blender preferences, look for
Addons, search for three, enable the checkbox next to the Import-Export: three.js format entry.
Goto Usage.
Should look like this:
C:\Users\USERNAME\AppData\Roaming\Blender Foundation\Blender\2.60\scripts\addons
Depends on where blender.app is. Assuming you copied it to your Applications folder:
/Applications/Blender/blender.app/Contents/MacOS/2.60/scripts/addons
By default, this should look like:
/home/USERNAME/.blender/2.60/scripts/addons
Use the regular Import and Export menu within Blender, select Three.js (js).