In the CruizePersonalizer click on Checkin
https://book.princess.com/cruisepersonalizer/paxCheckIn.page
Scroll Down and click on 'Dietary Requests'
Supply your email address, etc.
Then click on 'Food Allergy' and Select 'Milk' and in the comment section state you are lactose intolerant and would like lactose free milk. You will get an email back regarding that.