Once you have finished creating your Box account you will need to connect your Box account to your Salesforce account to be able to access the functionality provided by Box.

Log into your Salesforce account and navigate to the 9 dots on the top left of the screen.  Enter Box in the search field and click the Box application.

Select the Box Settings tab at the top of the screen. 

Next, Click Connect

You will be prompted to log into your Box account using your Box password and using your preferred 2-factor authentication method.

Next, you will be prompted to Allow or Deny Access.  Select Allow.

You will be redirected to the following screen confirming your Salesforce connection with Box.  You can navigate back to the 9 dots and search for the application you work in.