Which version of Sound Forge? Read this 'Sticky' at the head for the main Sound Forge forum page.

I am not not familiar with downloading Sound Forge purchased from third-party sellers, contact Best Buy.
Normally, one would go to the Magix Download Center or Magix Service Center > My Products. But since SF has not yet been registered, neither may not work, you need your serial code and a paired email address which is accomplished in registration. You can however download and install the 'Trial' version, which is the same as the regular version except for the activation serial code. When activating SF for the first time, the registration dialog should be available.

btw, welcome to the Magix Sound Forge users forum @bob-may and be sure to archive your serial code and the email address you use in registration, which are 'paired'.