explore the room. you'll find all the ingredients. then drop them into the pot on the blacony overlooking the "portal" on the floor. Serana will then bite her arm, drop in blood and the portal will open.
Thats when she weakens you if you make a certain choice.
When I went there, she walked around the room. I had found all but one of the ingredients and her mother's journal before she started talking about the portal. She started talking to me about it. I didn't click on her.