Download and install the Picasa Web Albums Uploader.
Within iPhoto, select iPhoto > Preferences and then the Accounts tab to connect to your Flickr account.
Within iPhoto, select the Flickr icon in your sidebar.
Within iPhoto, select the Flickr album you want to move and double click to open in order to make sure iPhoto retrieves all of the photos.
Within iPhoto, select File > Export and use the "Picasa Web Albums" tab to export your selected Flickr album right into Picasa.
Upon signing into your Google Plus account, and viewing your albums, you should now see the exported album available.