I have seen multiple Modifier Cheats for the US version of Pokemon Platinum, and there is only 1 that works EFFICIENTLY. The others either freeze, screw up gameplay, or just don't work at all. Although many of you would like to control the level of the Pokemon you are searching for, this cheat does not have that feature. You can try, but it will either result in freezing or failure.
Now, as with all Pokemon Modifier cheats, I will start with the beginning: first, make sure that this code is the only one in use on your Action Replay (Yes, you DO need one...). Mixing codes will most likely result in gameplay disruption. Second, move to a route that has grass. NOTE: Caves, water, etc, does NOT work. Once on a grassy route, switch to your Calculator App on your Poketch. Next, punch in the National Dex # of the Pokemon you are looking for. After typing in the number, press (do not hold) L. As with normal searching, you may have to walk through the grass for a while before encountering anything. Once you do, though, you can rest assured that it will be your Pokemon. You can catch/defeat the Pokemon with no glitches or freezing. I have even experimented with the legendaries and with Spiritomb, who seems to be a problem-pokemon when it comes to cheats.
NOTE: Maybe this is to discourage drastic level differences in the game, but you cannot control the Pokemon's level. However, when you encounter the Pokemon, it will be in the same level range of the wild Pokemon in that Route. For example, if you are searching for a Pikachu (#25) on Route 206 (under Cycling Road), it will range from levels 16-20. The gender and natures are completely random, but if you are so picky as to throw away a female Pokemon or a Docile-natured one, then just make your own cheat on PokeSav.
If anyone has any questions, please ask. I want to make sure that everyone can follow this cheat without any confusions or concerns. Happy cheating!!! ^^
PS: I may have accidentally submitted a similar cheat earlier, but the code is incomplete. If you see it, please let me know how to delete it (cuz I'm a n00b at this site and I have NO idea... <<;)