Playstation:
1. Start a 2 player game
2. Enter vs code 302 213
3. Exit 2 player game
4. Go to options
5. Highlight vs enabled button
6. Press and hold BL+RN until the cheat menu shows itself.

Nintendo 64:
1. Go to options
2. Highlight Continue
3. Press and hold BL+RN until the cheat menu shows itself.

Goro:
Step 1:
PSX: Beat game as Shinnok.
N64: Enable Cheat Code.
Step 2:
Select the Hidden button on the bottom of the select screen, move up 3, and over 1 to Shinnok, then press RN+BL together.

Noob Saibot:
Step 1:
PSX: Beat Game as Reiko, Start 2 player game, Enter vs code 012 012, exit 2 player game.
N64: Enable Cheat Code.
Step 2:
Select the Hidden button on the bottom of the select screen, move up 2, and over 1 to Reiko, then press RN+BL together.

Meat:
Complete group mode.

Note: these are done on the character select screen with the cursor around that character.
Hold Start+Press any action button.