Put script here C:\Program Files\Blender Foundation\Blender\.blender\scripts
Download 3ds2kexbeta to somewhere you know where it is,such that you can refer to it's location with Blender2.49b's Scripts Window:Scripts > System > Scripts Config Editor > Export.(See Post #19 in this thread).
Also check Windoze isn't running interference on either "export_3ds2kex.py" script or "3ds2kexbeta" by right clicking on them & checking properties,unblock them if it is.
It works fine on Windows 7 Enterprise & XP.
ps you want the 32bit version of Blender
NOT 64