Break up your lump of hamburger into smaller bits inside a bowl, one with a lid is ideal.
Cover your bowl with a lid or tight saran wrap and cook on 100% (normal) power for 4 minutes. Take it out, stir to breakup any lumps, and put it back in the microwave in 2 minute intervals (breaking up every time) for 4-5 more minutes or until browned.
The meat will be ready at this point but you should check internal temp. with a kitchen thermometer - should be 160-165F. Transfer the minced meat to a colander to drain excess fat.