I personally want to get a Munchlax so I found my Secret ID. The code I found from Pojo.com forum I felt it was not working. So I tried another code found on Gamefaqs and I believe it to work. The first code I found:
SECRET ID CODE (PRESS L+R)
94000130 FCFF0000
B21C4D28 00000000
B0000004 00000000
DA000000 000002E2
D6000000 000002E4
D2000000 00000000
This code is for DIAMOND/PEARL.