I lent my books out and my DDI sub ran out so I can't answer this basic question: do monsters get extra damage dice on crits like PC's do?
Generally no, they don't.
This is because almost no monsters come equipped with magical weaponry. And it is the weapon (or implement) that gives the extra crit dice.
Some (a few) monsters do have irregular critical damage, but then that's noted specifically right beside the power in question.
Of course, whenever you hand out a magical weapon to a monster to use before the players can loot it from its dead body, the monster should deal the appropriate extra crit damage according to the weapon.
Do take note of the "magic item threshold" rules of the DMG though! Or, if you're in a rush, the general quick-n-dirty rule is:
Don't add the enhancement bonus of a magical weapon to the attack and damage values of monsters and NPCs. And don't sweat it if you forget to use any special powers or properties of the weapon.
That is, the monster gets the weapon, but unless it crits, there is no difference in how it operates. Its only when the adventures loot the item that somebody will get the benefit of its enhancement bonuses.
This is by design, and not a mistake. It is supposed to work this way in 4E: monsters simply don't play by the same rules as PCs. And they should generally have worthless weaponry that in practice aren't lootable.