Okay, I'm not sure what Serebii posted, but if you're talking GameShark, I can help.
There are two ways to do this; the longer way (it takes more time), and the shorter way (it takes MUCH less time)
LONG WAY
Enter this code in the GameShark:
014089DA
Next time you talk to Kurt after putting that code in, you should be giving him the GS Ball. Wait one day and then go back to talk to him and he will talk about this not being an ordinary ball, blah blah blah, he runs outside, you follow him and he says "WHOAMG WAT IZ HAPPNIN IN ILEX FORUEST!?!!??!!1/!" but not really... He just tells you to go there with the GS ball, because Ilex is acting strange. The rest you should know, if you're trying to catch Celebi.
FAST WAY
Input this code:
01C089DA
This is why this will be faster than the way posted above, because the same thing happens, but you don't have to wait for 1 whole day. As soon as you talk to Kurt, everything from "He will tell you it's not an ordinary ball........" happens.
Happy Celebii catching, even if it is cheating.