pal park

That's simple.
If you have FireRed,LeafGreen,Ruby,Sapphire,or Emerald GBA games,put them into your DS's GBA Port.Turn on your game(Diamond),and when you get to the main menu,look for
'Migrate From(Insert game here).Select that,and choose which Pokes you want.You can choose six each day from each game.Hope this helps!

For this, you need a Game Boy Advanced game, which I'm guessing you already have.
1) Make sure the DS is OFF
2) Insert the GBA game into the slot on the DS
3) Turn on
4) Select the DS game when the opening screen comes on
5) Instead on pressing the CONTINUE button, there will be another option at the bottom of the screen that says MIGRATE FROM (YOUR GAME). Select that option.
6) Select YES, and then YES again
7) Pick your Pokemon Remember that you can only pick Pokemon from the PC boxes, not your current team. 8) Select YES, and then YES again
9) Select CONTINUE
10) Fly/Surf to Pal Park. (Surf south from Sandgem Town)
11) Talk to the guy in the hat and he'll send you into the Park. All you need to do is run around and find your Pokemon. *The Pokemon will be in their "natural" habitat in the park. Like Butterfree will be in the forest and Gorebyss will be in the ocean. So if you have water Pokemon that you're transferring, you'll need a Pokemon with the move Surf. Hope I helped.

Before I start anything, if you have a DSi, you cannot use it to put Pokemon in the Diamond/Pearl Pal Park. You need to use a DS or DS Lite. Now to be able to migrate Pokemon, you need to first have the National Pokedex, which I have an idea that you already have it. You need to have Diamond or Pearl in the DS slot, and a GBA Pokemon game (Ruby, Sapphire, Fire Red, etc.) in the GBA slot. Now go to the Diamond/Pearl main menu. You should see (for example) "Migrate from Sapphire". Now from there, it should guide you on what to do. By the way, you can only migrate a Pokemon if the Pokemon you have selected is in the game's PC.

Hope I helped,
Sk8rboy