To unlock the bonus games, you have to earn three star ratings on groups of five levels. The games are unlocked thusly:

Hard as Nails: Get three stars on each of levels 1-5.
Brawl Game: Get three stars on each of levels 6-10.
Monkey See, Monkey Die!: Get three stars on each of levels 11-15.
Time 2 Die: Get three stars on each of levels 16-20.