Experience keys are only available to Premium members. To acquire a key you must have an experience-enabled viewer, such as the official Second Life Viewer. In the Second Life Viewer:
Actually I'm having halloween thoughts, and looking into experiences because of the very cool things I could do, but blahhhh do not want to mess with all that LL stuff
I tried that, it's fun. You can make things like doors in your house that teleport you to a skybox. It feels as if if you just crossed into the next room.
Premium is required to avoid potential griefing use of experience keys, as I understand it. I still remember the day they released experiences earlier, and trolls figured out how to make a "teleportation gun."