Lab Rotation Placement
Assuming the department has been using rotations for a bit, I can't imagine this is such an unusual situation for the professor. Simply based on my own experience, I imagine that that most students don't reach out to the professor they're paired with prior to assignment. (I very well may be wrong here, but that's definitely my experience.)

Within that framework, I would read up on the professor's research and simply send an introductory email with an offer to meet at his or her convenience. They'll probably be able to walk you through expectations and requirements when you first meet.

If you're really nervous about violating social norms specific to this department, I would consult with the head of the graduate program and ask for their advice. They can guide you better than we would be able to in any event.