How do I open the Season Pass for Borderlands 2? I purchased the Season Pass through Steam. It says that I own it, but I can't open it.
The Borderland 2 Season Pass is DLC. It gets automatically applied to the base game when you buy it for yourself, so you should be able to just open Borderlands 2 and play.
In game, you've got the option "Downloadable content" in the main menu. This option gives you a list of all the downloadable contents available.
Your "unlocked" content is tagged with "Purchased" (first item on the screenshot)
It should install automatically to your base game which gives you 4 story DLCs.
"The Borderlands 2 Season Pass is packed full of adventure, featuring new campaigns, new locations, new enemies and more surprises."
Right click your Borderlands 2 game -> View downloadable content. If you see the DLC in there, you should be all set.
Use the fast travel system in game to spot the new destinations.
It does not come with all the DLC the game has to offer, so keep that in mind.