To get attuned for Naxxramas, you need to be at least honored with the Argent Dawn. With honored rep, a quest becomes available from Archmage Angela Dosantos in Light's Hope Chapel, Eastern Plaguelands. The quest name is The Dread Citadel - Naxxramas.

Requirements for attunement are 5 [Arcane Crystal], 2 [Nexus Crystal], 1 [Righteous Orb], and 60 gold.

At revered, the requirements for attunement are 2 [Arcane Crystal], 1 [Nexus Crystal], and 30g

At exalted rep, there is no charge, but be warned that it is a major hassle to get to exalted with Argent Dawn.

[Righteous Orb] is found in Stratholme dungeon, in the human side occupied by the Scarlet Crusade. Several guild members are currently farming this area to get the guild bank stocked up for anyone wishing to take part.