The password varies in each game. Mine was 9909, but others are apparently 2202 or 7707. It may just be a random digit in that pattern, xx0x.
Update: the password can also be Zekrom or Reshiram (in the English games that is, in the Japanese games I guess it's their Japanese names).
If you check what the plasma grunts say, you should get a clue. For me one of them said "2 [Japanese text] 9" and the other said "4 [Japanese text] 9" so I assumed the 2nd and 4th numbers were 9.