Glad you are exploring Tableau. That coupon code is the serial number. You simply download the trial version of Tableau from our website. It may prompt you for the code once installed. If not, you enter it on the Help, Manage Product Key. then choose the activate button. That should work.
If not, you can get help here:
There is a FAQ at the bottom and there is a link to an email address you can submit help for.
Hope that help.