You need to fill all 4 slots while also adding up to 55. The devs have mentioned adding more save slots in the future.
i got stuck on that for a while too; most walkthroughs show some slots being empty in the elevator fuse panel. in v.1.0.4 (in patch notes in the READ ME.txt) [+ Fixed an issue that allowed the elevator fuses to work without filling all the slots.], they fixed it so you have to fill all the slots. in the bloody operating room that you and Sai enter to escape the primal shadow demon, there's a locked gate that requires a key that you can find in the facility. when you unlock the gate and go through, there will be a fuse numbered 20 that you can use to activate the elevator. you should be able to figure it out pretty easily there, and don't remove the fuse numbered 25 with the door icon (or at least save the game before you do). sorry if it's too late though