This feature is not currently available for Picasa for Mac.
Using the Webcam capture button 
in Picasa, you can capture and import video or stills from within Picasa:
- Make sure your webcam is plugged in and fully operational.
- Click the Webcam Capture button on the top button bar. This will open the Capture box.
- To take a still photo, click the Camera button. To take a video, click the Record button to start and the Stop button to end. All videos and still photos from the Webcam Capture tool will be saved in the 'Captured Videos' folder.
You can adjust your video and audio recording tools, in addition to the size of the image captured, by clicking the Settings button in the Capture box.
If the Webcam Capture button is inactive or not present, first make sure your webcam is installed and plugged in to your computer. Then try closing down Picasa and opening it back up.