Does the tweaking cheat to get Darkrai and Shaymin actually work?

Yes it does, Majority of all the codes and stuff to get those guys do work.

Here are the ones I have personally used.
---Darkrai---

1)Use the Walk through anywhere code. Go to Full Moon Island and keep walking to the right until you encounter New Moon Island. Follow the path to get into the little patch where you have to fight and battle.

2)You can also use the Pokemon Modifier code and catch him any where. Look in the AR Replay Section for the Code.

3)You can receive the event item using code below to get him and follow the Event instructions under the Diamond/Pearl page on www.serebii.net
Darkrai Event - Press L+R
94000130 fcff0000
B21c4d28 00000000
B0000004 00000000
0000aa3c EDB88320
2000aa68 00000008
D2000000 00000000

4)After you Battle Darkai on the Isalnd you can use the code below to battle him over and over.
Re-battle Darkrai
94000130 fcff0000
B21c4d28 00000000
B0000004 00000000
20001273 00000000
20001293 00000000
D2000000 00000000

---SHAYMIN---

1)Go to the Castle that houses the Elite 4 use the walkthrough anything code. Walk to the right until you come to the Grass Patch beginning to the Flower Paradise. The Head North until you see Shaymin.

2)You can also use the Pokemon Modifier code and catch him any where. Look in the AR Replay Section for the Code.

3)You can receive the event item using code below to get him and follow the Event instructions under the Diamond/Pearl page on www.serebii.net

Shaymin Event - Press L+R
94000130 FCFF0000
B21C4D28 00000000
B0000004 00000000
0000AA3C EDB88320
2000AA68 00000009
D2000000 00000000


4)After you get him use the code below to get more.
Re-battle Shaymin
94000130 fcff0000
B21c4d28 00000000
B0000004 00000000
20001295 00000000
D2000000 00000000

Please ask this question in the POKEMON AR FORUM.

Most of the time, yes.

But how do you put those codes into the action replay???

Look on YouTube to learn how to get Shaymin by tweaking. No AR codes, events, nothing!