Yes you can get Manaphy
Put your game Pokémon Ranger into your DS system and turn it on
Complete the Continue Missions in Pokémon Ranger. Afterwards, you should have a new feature on the title screen; Ranger Net.
Hold in the R Button, the X Button, and Left on the D-Pad. The option Enter the Password will appear under 'Select a Mission'. Tap the option.
On the Password Enter screen, type in Mg35-Cpb8-4FW8.
On the Select a Mission screen, the mission "Rescue the Precious Egg!" will appear. Accept this mission.
After completing the mission, Check the Egg will replace Enter the Password on the Ranger Net menu.
Once you're ready, put Pokémon Diamond or Pearl in the second DS. Turn it on, and go to the Main Menu (Continue, New Game, etc).
In Ranger, tap 'Check the Egg', and tap the screen. Afterwards, tap 'Send the Message', and place the 2 DS' across from each other.
In Diamond or Pearl, a new screen should come up. From here, follow the instructions provided.
Congratulations, you now have the Manaphy Egg in either Diamond or Pearl! To hatch it, go to any PokeMart (not the department store or Pokemon league) in the Sinnoh region and talk to the man in green. He'll give you the egg (provided that you have an open spot in your Party).
Walk roughly 2,740 steps. After that, it should hatch