Log in to your Profile and click on your Dashboard. Click Join 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. With this the the total fee (Due at Checkout) 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