McMMO has nothing to do with PopularMMOs. McMMO is a plugin that offers upgrades or powerups based on a certain activity. You can get McMMOS for many different things. You get McMMO levels by doing the activities and leveling up (It will tell you on your screen when you level up). The higher the level, the more upgraded in some way the McMMO powerup will be.
Ex. Fist McMMO: Unnarmed ~ While attacking with your fist, you have a chance to steal the other player's item that is currently in their hands.