How do I access the "The Secret Armory of General Knoxx"?

As with all the DLC expansions from Borderlands, The Secret Armory of General Knoxx is accessed through the quick-travel system.

All you have to do is locate your nearest quick travel, and travel to T-Bone Junction, the DLC starting area.

Keep in mind that there is some assumption¹ that the player has completed the main game. If you have not, you can still access the new area, but scaling can be rather particular. It might be a breeze, it might be a death walk².

_{¹ It is clear, from introductory dialogue, that the DLC is designed to take place after the opening of the vault, however, you should not have a problem accessing the DLC prior to completing the main game.² Not sure how the scale works, but I was able to roll through the Halloween DLC as a brand new level 1 character, only to have a level 10 character stopped in the first area. IIRC, the armory was more difficult, in terms of scaling to the player's level.}