Printer Setup Utility gone in Mavericks, how to set default printer in Applescript now?
You can use AppleScript to execute shell scripts, so you can use:

do shell script ("lpoptions -d printername")


Replace printername with the actual printer name. It may not be exactly as it shows in System Preferences (I think ALL non-alphanumeric characters are replaced with underscores, but I'm not certain) so you can view your list of printers with:

lpstat -p