The problem is Apple flat-out blocks Flash, at least the older versions that work with Poser. I don't think there's any way around it.

I think buying Poser Debut was a mistake. You should upgrade, not downgrade. Buy the new version, not an obsolete version. Especially since you have a Mac, which isn't as open to tinkering.

You can try the newest version of Poser for free:

https://www.posersoftware.com/downloads

There's a 21 day free trial period, and if you like it, you can get upgrade pricing, since you own older versions of Poser.

Alternately, you could try DAZ Studio, which is free, and has a reputation for being user friendly.