In order to use Picasa's face recognition feature double click on an image that you would like to edit. It will automatically identify each person. Then label the first person that it identifies. Then hit the enter key, and click on the new person button then click on the okay button. Now go back to your library and on the left hand side of your library you should see the label that you made for your first photo. Click on that label and Picasa will automatically find all of the photos that that person is in in your library.