It's just a short dialogue that should play automatically in her room if you visit her after reaching relationship 5 and having seen the dream.
After that short dialogue scene, you need to go to the basement in the morning.
Provided it works as intended, the hints for optional content on her info page should also tell you what to do next, though it depends a bit on whether you've already cleared any "higher priority" side content before the dungeon hint shows up.
If this doesn't help, you could try uploading your savegame on our discord so we can check whether or not there might be a bug.