Entering the Code
While an up-to-date Action Replay DS already has many codes for "Pokemon Diamond Version," you'll notice that it doesn't have a wild Pokemon modifier code. To get this code on your game, you'll have to enter it manually. Turn your DS on with the Action Replay inserted and navigate to the "Pokemon Diamond Version" code list. Select "Add New Code" and name the code. On the next screen, enter "94000130 FDFF0000 B21C4D28 00000000 B0000004 00000000 94000130 FEFF0000 00000890 01ED0001 D0000000 00000000 DA000000 00000892 DC000000 00024620 C0000000 0000000B D7000000 00000000 DC000000 00000006 D2000000 00000000" and make sure you get every single character correct or the code won't work. Enable the code and start your game as you normally would. Once your game starts, press the "L" and "R" shoulder buttons at the same time to activate it.
Pokemon Math
While the code doesn't directly give you Ho-Oh, it gives you 493 Master Balls. There are 493 Pokemon in the "Pokemon Diamond Version" National Pokedex, meaning you have 1 Master Ball for each creature. To get a specific Pokemon, you need to subtract a certain number of Master Balls so that the number remaining in your inventory corresponds with that Pokemon's number in the Pokedex. Since Ho-Oh is number 250, you'll discard 243 Master Balls, leaving you with 250.
Catching Ho-Oh
The code forces an encounter with Ho-Oh, where you'd normally encounter a random Pokemon. Walk into a grassy area and move around until you're automatically entered into a battle with a wild Pokemon, which will now be Ho-Oh. Wear Ho-Oh down as you would any other Pokemon, leaving enough HP so he doesn't faint, but taking enough away to easily capture him. Alternately, use one of your leftover Master Balls to catch him. Master Balls are a type of Pokeball that always catches the creature it's thrown at, regardless of HP or difficulty.
Alternate Code
If you'd rather not deal with subtraction and Master Balls, an alternate code cuts to the chase and lets you specifically catch Ho-Oh. Enter "94000130 fffb0000 d5000000 000000fa b21c4d28 00000000 b0000004 00000000 c0000000 0000000b d7000000 00024620 dc000000 00000006 d2000000 00000000" into your Action Replay, activate the code and start your game. Go to a grassy area and hold "Select" while walking through it. The first Pokemon you encounter will be Ho-Oh. After catching Ho-Oh, save your game and restart, disabling the code so you can once again encounter random wild Pokemon.