You can't. Legally, anyway.
The noble map pack.
Two multi-player map packs. Noble Map Pack and Defiant Map Pack
You buy the map pack at marketplace or buy halo anniversary
you cant. Microsoft has made it impossible to get the legendary map pack for free.
there are three
Defiant Map Pack
March 2011
No, I'm sorry but you cannot spawn computers or laptops in halo reach. You may be able to on some of the Noble Map pack or Defiant map pack maps as I have not played them, but I doubt it as well
The Heroic Map Pack for Halo 3 is now free. You can download it via xbox live marketplace.
NO but it has new maps the new defiant map pack has maps and 1 firfight map also
== == The map pack is free now, but the Legendary map pack costs 800 MS Points == ==
yes it will be soon