altium V6 is quite old. Have you tried (and had success) with importing some newer altium PCB and schematic? This can at least confirm you’ve pushed the right buttons in KiCad to do the import (I think only import of schematic and PCB works, not the project themself (yet)).
One thing you can do is ask someone who has altium to open and save your project in an attempt to port it to a more modern altium version. I think altium also has a 2 weeks “free trail” version at the moment.
Also, this is much less worse then it seems at first sight:
The footprints themselves really are not that valuable for a conversion, and you get much more then the tracks. You also get the PCB outline, and you get the locations of all footprints. This means you do not have to redo the placement of the footprints, but can simply put them on existing track ends. Are you aware of