Thx Lyndel. I was able to get it working later that night actually. Been meaning to come back here and post in case anyone else stumbles onto this, who knows maybe it will be me when OSX 10.9 comes out and I spend another evening trying to get everything working! It's possible some of this isn't needed as I tried a *lot* of different approaches but here's what I did:
1. my serial-usb adapter is the Keyspan USA-19HS. I installed the 2.6 version of their driver which does work in Mountain Lion. You need to reboot for this to actually detect the adapter.
2. at some point I had installed Java 7, so I then followed the instructions at [Link: support.apple.com]
to drop back down to 6. Not sure if this was necessary.
3. temporarily had to disable OSX app security (System Preferences | Security & Privacy | Allow applications downloaded from: Anywhere) so that I could run the jnlp which has an expired certificate.
4. went to [Link: mrallen.com]
and saved the jnlp for installing with Java web start. Ran this to download the app (remember where you save it), and then quit.
5. right click the app icon, and set it to run in 32 bit mode
6. re-enable OSX app security
So now I can just doubleclick the app (it always asks to re-download, I just cancel out of that), and I've been able to pull ccf's off the Pronto and put modified ones back on. Took me a few hours, but glad that my favorite remote still has some life left in it!
(I had also tried running Java from the terminal with various jars I found on mrallen.com, variations on:
java -d32 -cp tonto.jar:libs.jar Tonto
but would always get error messages, presumably b/c it couldn't ever find the JComm lib. Tried lots of different ways to get at it but no luck...once upon a time I was a Java developer but it has been years since it was my day-to-day language and I never used JNI so am not so hot at troubleshooting it!)