So lately I've been trying to use the GECK in order to add a room to the Hidden Valley Bunker so I could make my own custom home/room, but every time I open ANY interior world spaces, they never show in the rendering window, making it impossible for me to be able to accurately create my place. Is there any ways that anyone knows how to fix this? I'd really like to be able to use the GECK again. It never did this before.

Have a look at this thread, http://www.gamesas.com/topic/1495567-geck-questions-from-ck-veteran/

Only issue I've had with this is loading up exterior landscapes gives me CTD so I have to keep changing it back for that.......it's annoying!