If you have the ability to go back to Lion, I would do that ASAP! I had a Lion backup (SuperDuper) on an external USB drive, and foolishly did not retain it, instead I backed up my Yosemite installation. HUGE mistake. So I am cut off from going back.
I could explain if we were sitting down together at a table with the computer in front of us, but probably not via email. Basically, you need to be comfortable using the Terminal (In Finder: Applications | Utilities | Terminal.app). It is a command line interface, no graphical interface. You type (or better yet, copy and paste) commands, enter your password, say a magic incantation (just kidding but it doesn't hurt) and press return. If all goes well, the command will execute; if not, you risk causing all kinds of problems with your computer, which is why I say it is not for the faint at heart! You have been warned!
Assuming you have the Terminal open, copy this and paste it to the command prompt:
sudo nvram boot-args="kext-dev-mode=1"
The "sudo" makes you the superuser (administrator) and the command that follows allows unsigned drivers to be loaded.
Now you would install your driver. I used the Lion driver from Miklor.com and it worked at the beginning.
It was necessary to manually load the driver the first time with another command in Terminal (again, copy and paste):
sudo kextload /System/Library/Extensions/osx-pl2303.kext
It worked for a while, then although the driver continued to show up in CHIRP, it always returned the error message "The radio did not respond." Same radio, same cable, work fine on my 2006 white Macbook.
Like I say, if anybody gets CHIRP working reliably in Yosemite, they need to detail how on a Miklor page!