how do I edit a .kext file in snowleopard 10.6.8.

I have a usb to serial converter uc-sgt. which uses the prolificusbserial 2030 chip. There is no specific driver for this device but prolific make a generic driver. I found a tutorial on how to edit the .kext file and insert the correct hardware and vendor id to make the generic driver work for this converter. But snowleopard 10.6.8 will not let me overwrite the .kext file I keep getting the error "this file has been installed improperly and will not be used" What can i do.

all the other tutorials related to kext problems suggest reverting to a previous version but that is not an option in this case.

thanks for your help.