Make sure you download the DLC from the in game menu. After that, each should be available to travel to from any fast travel station at any level. Oasis, Badass Crater and Hunter's Grotto are the 3 areas.
If you have them downloaded you will fast travel to them. Oasis (Captain Scarlett and Her Pirate's Booty), Hunter's Grotto (Sir Hammerlock's Big Game Hunt) and Badass Crater of Badassitude (Mr. Torgue's Campaign of Carnage) are the locations you are looking for.