The COAA website is asking me to pay a registration fee. Is that right? Also, I don't have credit card. So, I can't pay the $30 they want in order to register the program.
No idea how the website works
Did I mention that you can contact Bev (the original and main developer) and that there is a support group?
I have nothing to do with the software except that I have been using it to feed ADSB data to their system for a very long time. I don't use it as an ACARS decoder (though I did test that fuctionality a very long time ago and it worked fine when fed audio from the radios I tried it with). I use other things specifically written to decode legacy ACARS so don't need PP for that.
So, Which software program do you suggest?
I don't -there are a large number of choices. You discounted one because it is 'abandonware' (though it decodes legacy ACARS messages just as well now as it did in the dim and distant past when it came out). Presumably that means you'll discount pretty much every other audio decoder as none are actively developed any more (why would anyone bother when legacy ACARS is old technology that is being phased out?).
The newer ones were almost all written for linux to leverage cheap hardware like the raspberry pi and rtl devices - but you have also said you aren't interested in that route.