First you'll need to set up the script on the Mac.
The python script you'll need is attached to this step. Download it, rename it slideshow.py, and make sure you save it to your home directory. There are comments in the file to explain setting it up, but there are a few things you'll need first:
1. The Google Data Python client. Just go to http://code.google.com/p/gdata-python-client/downloads/list
and download the latest version (at the time of this writing it's 2.0.16). Extract the files and open up the folder it extracted to. Once in there go to the 'src' folder and copy the two folders there (gdata and atom) to your home directory. See images.
2. A Dropbox folder for your pictures.
3. A Picasa album to put your pictures into. (Albums must contain a picture so just upload a dummy image that you can delete later)
The only change to the python file are adding you album name and Dropbox folder location.
After you make the changes to the script you'll need to open Terminal to run it. (You can find this at /Applications/Utilities/Terminal.app) Type "python slideshow.py" and hit return to run the script. First it should ask you for you Google email and passord. Then it will say "Photo booth started!" and then just sit there waiting for new pictures. When a picture is taken with Photo Booth, it will say "New pics... ...combined... ...uploaded!"
You should be able to test it at this point by taking a 4 picture set with Photo Booth and then looking in your Dropbox folder and your Picasa album for the pictures to show up. When you're actually using it at the event, you can press cmd-h to hind Terminal. When you're done with the photo booth you can just close the terminal window and the script will stop running.
The last thing you need to do is set your computer to never go to sleep. Go to System Preferences -> Energy Saver and change both "Computer Sleep" and "Display Sleep" to "Never."