Quints - Armor Penetration
Reds - Armor Penetration
Yellows - Health per level or Dodge Chance
Blues - Flat Magic Resist or Per level Magic Resist
0/21/9 - This is for your standard tank Mundo, Get tank masteries in defense, and in utility get the improved HP regen, ghost (if you use ghost) and EXP boost.
21-0-9 - This is for AD mundo, Get standard carry masteries in offense, and in utility improved HP regen, ghost and EXP boost.
Get Dorans shield first, secondly get a spirit visage, and this is where you build differentiates if you're playing AD or Tank.
I usually build him as a Tank/DPS, Getting items like Atma's, Sunfire cape, and Starks. Otherwise I get Randiun's omen.
For AD, max your E first.
As Tank max your Q first.
As Hybrid Tank/AD get Q First, and max E afterwards.