Please help, how do I use doors????

[Leon 1⋅1]

They don't work, no matter what I do

[Jaggid Edje]

You need to explain better...

Staff buildings have doors on them, and staff will go in and out of the buildings through those doors without you needing to do anything.

If you mean the "doors" that are in construction pieces, those are just cosmetic and not functional.
If you put them on pathways, visitors and staff can go right through them like they aren't even there though. So you can use them to give the appearance of working doors. They do not, however, open and close.
If you put them in a habitat, however, neither the animals nor the keeper can pass through them. So don't use them in that way.