Here is a guide I found a while ago, I hope it's what you're looking for.
Step 1: Access Ranger Net after finishing the DS game Pokemon Ranger. When you see ranger net on your main menu, click on it.
Step 2: Your bottom screen will show “Play a special mission.” On your Nintendo DS press the top Right trigger, the X Button, and left on your directional + pad, all at the same time.
Step 3: You should now see a screen that says ” ‘Enter the password’ has been added.”
Step 4: Tap the touch screen, and wait as the game saves.
Step 5: After the game saves, tap the screen again, and a new menu will come up that says “Enter the password.”
Step 6: When you click on the password menu, you’ll be allowed to “Enter the password”!
Step 7: Enter the password with capital letters:
* In America / Canada / Australia it is P8M2–9D6F–43H7
* In Europe it is Mg35-Cpb8-4FW8 says www.manaphy.eu
Step 8: You’ll see special message that says the mission is active “You’ve been assigned the special mission: ‘Recover the Precious Egg’ Play it from the Ranger Net menu. Please turn off the game.” Turn off the game.
Step 9: Turn on the game again. At the menu screen, access Ranger Net and you’ll see an option to “Play a special mission”. Select it.
Step 10: The mission “Recover the Precious Egg!” appears. Select it.
Step 11: You did it! Congratulations, now start the mission to get the Manaphy Egg.