Most of our plugins use licensing to help facilitate plugin auto updates. The plugins will still work without license activation, so there's no need to activate a license on a staging site unless you just want to verify that the activation works or if you want to download available updates to the staging site.
Once you have activated a license, you can manage your license(s) on the My Account page. Accounts are automatically created upon checkout, so just because you didn't create an account, doesn't mean one doesn't exist. You can use the "forgot password" link on this page to set your password for the account. The email address to use would be the one the confirmation email was sent to.
Some older orders didn't get accounts created for them, so if you attempt the Forgot Password and it says you don't have an account, send me a message through the contact form, and I'll get you set up.
If you updated a plugin and saw the activation notice for the first time, you may need to disable the plugin and reactivate it before the license activation works. The error people usually receive in this situation is "Invalid Action".
If you're seeing an "Invalid License Key" error, it's possible that you're using the wrong email address. While the email address that shows up next to the license is typically the email address that was used to order the plugin, if the email address associated with the account has been changed, then the email address that is currently associated with the account is the one you want to use.