According to http://dougscripts.com/ setting shuffle and repeat modes via applescript is broken in iTunes 11.
According to this stackoverflow answer shuffle is now a
Here is another approach:
activate application "iTunes" tell application "System Events" tell process "iTunes" click menu item 1 of menu 1 of menu item "Shuffle" of menu 1 of menu bar item "Controls" of menu bar 1 end tell end tell