For whatever reason, the 'Doors' check-box in the Show/Hide window of the Creation Kit is grayed out, along with 'Helper Objects' and 'Activators', and I can't figure out why. It's preventing me form creating a teleport door and its corresponding teleport markers. They were accessible previously, but they aren't now. Did I maybe change an imoportant setting by accident, or what?

If you don't see the markers, hit M to toggle markers. If the actual door is invisible, then I'm not sure what's going on. :/

I've got markers tuned on, and they all show up, except for the DoorMarker one. I can't add a DoorMarker object to the cell, either, when the 'Doors' option is grayed out.

I just checked, those three are greyed out for me too.

You don't add doormarkers to the world, you check 'Teleport' in the doors reference window, then tell it what door to connect to. Then Tele markers appear on both ends.

Oh, that explains that. I somehow managed to forget about that in less than a week... Thanks for the help...