How to install fax software onto iMac G4 (OS X 10.3.9)?

I am a computer engineer and I will love to help you on this. As mentioned in your question that you get the message that the destination folder locked. One thing I want to make you clear that this type of message like you have not got the permission to modify the computer or the message that you have mentioned always occurs due to the type of user at the time of logging of a computer. This always happen when you log in a computer as a guest user or any user except the administrative use. So for the solving of this problem make sure the ID that is used by you in the process of log in the computer must be administrative. The administrative user have only got the privilege and the authorization to install any software in the computer or make any changes in the computer. Hope after logging in the computer as a administrator your problem will be solved completely and you will be able to install the desire software.