Are you referring to the scene where you see her going into the Torture Rack? Because I already did that - that's how I got the option to talk to her about being a domme in the first place. If you're talking about something else, then I have no clue where I'm supposed to find her.
Hmm, there should be an option for asking to be her subbe when you talk to her in her condo. Though it only appears if you have female partners active.
...That was my problem. I didn't have female partners active. I was used to playing the original version, which didn't have a way to disable content like that! I'm a dope. Sorry, and thank you! ^_^;