Character mistake: General Landry says that the Asurans would need to rewrite McKay's macro to gate to any planet in the Milky Way. In fact they wouldn't have to bother with the macro, because they could simply gate directly to the Milky Way from a ZPM-powered Atlantis.
Continuity mistake: Lt. Colonel John Sheppard is packing his belongings up and his skateboard is against the wall at first, then it is on top of boxes.
Answer: Because even all 4 Naquadah generators (one was destroyed) together is only enough to dial Earth for approximately 1.2 seconds (As seen in the episode "Letter from Pegasus"). Further, if Earth had found a ZPM, they would dial Atlantis and let them know. After all, the SGC has no other use for a ZPM than to dial Atlantis.
Grumpy Scot