No one should ever develop software that requires Flash anymore. Every company is dropping support for Flash now because it's a portal for malware on most systems, including Apple. It's mostly the Flash Player and online browser support that is the issue, but I seriously wouldn't trust anything that depends on Flash to run anymore. There's so many better options out there for dynamic software programming.

Either use it on an old machine that will support Flash, or upgrade Poser. That's really the only alternatives you have now. Running flash on modern OS that is connected to the internet is asking for trouble.