Pat - there was a post on this group last month regarding this and the solution:

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Here is what I found. The driver is the issue with Yosemite.  The built-in FTDI driver prevents the osx-pl2303 driver from loading.  The fix (not for the faint of heart) is to do some black magic at the command line.

In Terminal, enter:

sudo nvram boot-args="kext-dev-mode=1"

This allows other drivers to load.  The driver that works for me is the Lion driver from miklor.com.