This only works if you used the code to get the 4 original zombie maps that came with the Hardened and Prestige editions!

What you do is really quite simple, you go to your download history by hitting Guide button(center X button)>Account management>Download history. Go through it until you find -Rezurrection Map Pack 4 [EN]- (It'll be something like that, probably not exactly), just click on it and download again. I'm assuming the reason is because Treyarch/Activision won't make people pay 1200 15$ for 1 map, because the other 4 maps including in this map pack are the 4 original zombies maps, that you already have,