From your 'My Hub' page (please see below)
On the left hand side of your account, you will see the side menu in which you can select 'Marketplace'.
Under the 'Marketplace' you will be able to view and purchase any packages your trainer has created.
Simply click the package you want...
Whilst filling in your card details you will also have the option to add a discount voucher (If your trainer has offered one) just remember to click the 'Apply' button once entered.
Select the 'Buy Now' button at the bottom of the page where just above that you will see the reduced price.