The Disney code to make park reservations in Orlando can be found on your voucher, under the barcode, and consists of 12 figures: 4 letters followed by 8 digits. To access the Disney parks, a valid ticket and date-specific reservation are required on My Disney Experience with your confirmation number.
To link your Walt Disney World theme park tickets, you will need to sign in to your My Disney Experience account (or create one if you don't already have one), then go to "My Plans." Near the top of that page, you will see an area to link tickets. Click on the button to "Link Tickets" and you will enter your confirmation number on that page to link the tickets to your account.
If you are still having trouble, you will want to give Disney a call at +1 407 939 4357, or if you are calling from the UK 0800 0280778 and speak with a Cast Member.