Instead of trying to force your cat to do something she clearly doesn't want to do, what about finding a creative solution for a litter box in your tiny apartment?: http://reclaimedhome.com/2013/09/16/...-litter-boxes/
Or is there a closet or cupboard that they can use?
Here are a couple links by renowned cat behaviour experts explaining why toilet training cats is generally considered a bad idea:
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler