If you stick to the usual definition of Work = (force) x (distance), then the answer
is zero. The microwave oven doesn't exert any mechanical force on the leftover
meatloaf, and at the end of the 5 minutes of cooking time, the meatloaf is in the
exact same place where it started. So 'force' and 'distance' are both zero.
But if we recognize that 'work' is just a transfer of energy, then we can easily
calculate the cooking energy that the oven pumps into its cooking chamber.
Remember that 1 watt = 1 joule per second.
(500 joule/sec) x (5 min) x (60 sec/min) = 150,000 joules