How muck work is done using a 500 watt microwave oven for 5 minutes

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