Firstly you want to make sure which version of the software you have purchased. Download or Online
However, if you purchased the online version, then you sign into your TurboTax online account, choose the edition you purchased, then at the end of your tax return on the payment screen, you will be prompted to enter your activation code, provided in the email.
Please note if you are using a MAC computer or a phone, you will not be able to use the downloaded version, you will have to use the online version instead.