take a cup or so of water out of your tank and dissolve the salt (1 teaspoon per gallon) in that water. Then every 15 minutes or so add a bit of water back into the tank until all the water is back in the tank.
unfortunately since this is severe dropsy, there isn't much hope to save her. If this goes on for a few days and she isn't getting better then I suggest you look into euthanizing her since she will be suffering from dropsy.
No. Aquarium Salt is not the same as Epsom Salt. Aquarium salt should be used in the case of fin rot or other physical injuries to the fish. Aqarium salt will make the dropsy worse. You need Epsom Salt. I suggest buying it asap. Meaning go to the store right now. The faster you treat her the more chance you have of her surviving. If all your stores are closed at the moment then go first thing in the morning, wake up early if you have to.