When you are on the booking page it will state the following;
If you are a Kew Member, please select the 'Kew Member' option below. You will need to enter your membership number in the promotional code box when prompted before purchasing tickets.
You will be given the option to select the tab that says 'Kew Member? Buy tickets here'
Kew Members will be asked to supply their membership number at the point of booking to qualify for their member discounts. Members may purchase up to 14 tickets per household.
If you have any issues with your membership, please read here.