Please use @mpepping's fork instead: https://github.com/mpepping/osx-pl2303/
I've been a deadbeat maintainer of this fork for a few years now. Sorry for the inactivity.
Good news, though: @mpepping has agreed to keep moving the fork forward, and will almost certainly be a better maintainer than I've been. What was here has been moved to the old branch, so it's still around, but please see the fork for any updates.
Thank you to B.J. Arnoldus for writing the driver in the first place, Michael Haller for patching the driver to work under SL, and the various folks who contributed patches to the Sourceforge tracker, some of which I've applied here, and finally to Martijn Pepping for taking over this unofficial fork of the driver!
-Bryan