InstallerNG has some additional features which may not be compatible with some installation scripts.
Have you run snoopdos to see where that particular installation script is looking for the installer tool?
You do not need to either activate it or do anything else. As long as it is there, it should work.
To cover all bases I sometimes copy "installer" to c:, utilities, and tools (just to be on the safe side).
Installer can be found on your OS installation disks (OS3.0/3.1 - on the "installation" disk iirc).
Regards
Rich
EDIT: Updated typos - had put "install" instead of "installer". Thanks PJHutch.