There is also another way to get manaphy and phione but it's a little time consuming. If you have the first pokemon ranger game there is a special wifi mission involving manaphy and once you complete it you are able to transfer a manaphy egg from that game to pokemon diamond pearl and platinum (will need two ds systems for this). Run around until the egg hatches and then put the manaphy that hatches into the day care with a ditto and then you will get an egg, run around again for a while and then phione will hatch from that egg. Source: personal experience
