This feature is only available to Premium users. If you don't have a premium account yet, you can still install it. You will be offered a free two-week trial when you first try to use it. You can also purchase a Premium license from within the add-on.
ProWritingAid uses your google email address to see if you have a premium account. If your premium account is associated with a different email address then read how to change your account email address.
Read how to change your account email address
Once you've installed the add-on it will appear in the Add-ons menu in a Google word-processing document.