Log in to your Profile and click on your Dashboard. Choose Become a member on your preferred clinical division. Follow the instructions until the end; before you click on Process Payment, at the right side you will find a Discount Code field. Fill in your membership voucher code, click on Apply, and the total fee changes. Click on Process Payment and the process is finished.
A step-by-step guide can be accessed here (login required): How to activate memberships with vouchers