Hey! Here are some awesome cheats for Pokemon Diamond. There are no action replay codes though, these are actually some tips and other stuff for the game.
This is how you catch Rotom.
1. Go to Old Chateau.
2. Go to upstairs room with tv.
3. Save turn ds off. Now turn your ds on and change the time to midnight (00:00) Turn off ds.
4. Turn ds on again, then enter game and go up to the tv.
5. Now, press "A" in front of it and if it says that a Pokemon will thump out, then Rotom should come out.
If it doesn't work, then repeat stept three through five but start where it says "Now turn on your ds." Rotom might be at level 15 so be careful not to make it faint so you can catch it.
There are three ways to get the odd keystone. Obviously there are also 3 odd keystones.
Way 1: Go to the bottom of Twinleaf Town, use the move "surf" in the lake and press "A" somewhere between the trees surrounding it and in a few seconds you will obtain an Odd Keystone.
Way 2: Go to Route 208 near Hearthome City and the Berry Master's place and there will be a karate guy hiding near some trees. Go talk to him and he wil say, "Don't ask. Just take it." Then you will have the Odd Keystone.
Way 3:(Hard way.) Just go underground using your explorer kit (obtained by a guy in Eterna City) and search all over the place until you find it.
These are the ways to get all the Eevee evolutions.
Vaporeon - Eevee + Water Stone
Jolteon - Eevee + Thunder Stone
Flareon - Eevee + Fire Stone
Espeon - Eevee + Happiness + Level up (Day)
Umbreon - Eevee + Happiness + Level up (Night)
Leafeon - Eevee + Happiness + Level up at Moss Rock in Eterna City
Glaceon - Eevee + Happiness + Level up at Ice Rock in Snowpoint City
How to get just normal Eevee: You must beat the entire game and have the national pokedex then, go to Hearhome City and talk to Bebe.