I wonder how many people ever actually ran into this problem:
The Newton Toolkit v1.0.1 and v1.5.1 are not compatible with Power Macintosh 9500 computers. There is an updater available from online services and the Internet which updates the Newton Toolkit to a beta version of 1.5.2. Newton Toolkit is a Newton software development program available through Apple Programmers and Developers Association (APDA).