Ok, I will tell u how to get dawn to say Happy Birthday. The best part is, no cheats/hacks are required! NOTE: You must beat the Elite 4 and get the National Pokedex.

First, reset your DS to the birthday you set on the DS.
Second, talk to Dawn/Lucas and voila! They will grant you a Happy B day. This might not work so do not, and I mean DO NOT! Blame this on me. This remains untested.