In order to unlock Pinky, you have to play as the pac chicken. In regular gameplay you have a chance of seeing some cherries on the ground. Pick one up and you'll unlock Pinky!
In order to unlock Clyde you have to play as pacman. When you eat a power pellet, you have to eat 7 ghosts in order to unlock Clyde. (this HAS to be done with one power pellet)
In order to unlock Blinky, you have to play as pac chicken. As you play, you might see a car lane with no cars in it. Just wait in that lane and pacman will come out of no where and gobble you up. This will unlock Blinky!
In order to unlock Inky, you have to play as pacman. You have to eat a total of 2500 pac-dots. This can be done across any number of rounds.