Jade

To find and fight Jade, wait until just before the "?" match in a one-player game. During this match, you must win one round using ONLY the Low Kick button and the joystick. You must not press any other buttons. If you did the trick correctly, you will be transported to Goro's Lair, and you will fight Jade there.

Smoke

To reach Smoke, you must fight a match on the Portal stage. This can be done during a one or two player game. As the match progresses, keep your eyes open for MKII's Sound Designer, Dan Forden. He'll make a brief appearance in the lower-right corner of the screen and will say "Toasty". While Dan's face is on the screen, hold the joystick Down and press Start. If you did the trick correctly, you will be transported to Goro's Lair, and you will fight Smoke there.

Noob Saibot

To find and fight Noob Saibot, you must win 50 matches in a row in two-player mode. (You only have to win 25 matches in a row in the Sega Genesis version) If you did that, you will be transported to Goro's Lair where Noob Saibot will be waiting for you.