Not quite sure how your query relates to lounge rules?
You need to log into Cathay's Manage my Booking, you'll need your PNR and name generally speaking to do so.
To do this, head to Cathay's site, then Manage Booking, then continue as a guest. Enter the above info and you should then be able to access the booking. The option to add a frequent flyer number should be under your name.
Failing that, you can just show your card at check in/at the lounge so they can attach it for you.