How do I access my SHiFT code golden keys in the Pre-Sequel?
Solution 1:
You have to visit the Shift station (across the square from the Fast Travel station in Concordia) in order to claim your golden keys. Golden keys are still stored in your global profile, so keys redeemed on one character are available to all of your characters.
Solution 2:
In the Pre-Sequel, Shift key redemption has become a multi-step process.
I'm expanding on the accepted answer and other comments with a complete walk-through.
Enter a Shift code. Orcz maintains a good list. You can do this in the game's main menu at Extras > Shift Code (menu option) > Shift Code (button). You can also do it in-game as described below.
Get to the "town square" in Concordia. If you begin at the fast travel console in town square, you can use the jump pad directly in front of you to reach the next location: the Shift kiosk.
Visit and activate the Shift kiosk in town square. This will show you a screen that (currently) just promotes Borderlands social media sites. But if you look up, you'll see a tab-button with a key and a gear as shown below. Click it.
Enter keys in-game if you wish. Now you're on the golden key management screen. If you haven't entered Shift codes on the game's main menu, you can enter them here.
Activate the redeemed keys. You should see one or more keys on the left; the screenshot above shows one. Click a key to truly activate it.
And now (finally) you can spend your golden keys at the mysterious chest in town square. I imagine Gearbox made the process more cumbersome for a reason, but it's not apparent to me.