Unfortunately, this software doesn't work with the latest version of iPhoto since it became a 64-bit app, probably starting with version 9.5. Using iPhoto 9.5.1 on Mac OS X 10.9.2, the Picasa option no longer appears in the "Export" dialog, and a corresponding "no matching architecture in universal wrapper" is written to the console. I confirmed via command line that the iPhoto 9.5.1 app is a 64-bit executable, and the plugin (version 1.5.0.1325) has only i386 and ppc architectures.
It's really a shame that Google doesn't support this anymore. :(