Acroma, the first thing that I suggest is to uninstall IconPackager, reboot (that is an important step) then reinstall.
The icon pack that you listed is an .ip, and it functions correctly. However, since you sound like you are getting that error a lot, it is probably due to a bad install (or upgrade) of IP.
Also, you can try adding the pack from within IconPackager.
What version of IP are you using? What version of Windows are you using?