go to NI website, login and you should see the registered software that has been transferred to you from the other user. you should be able to download the product from your account.
install product. activate product with serial number.
now it should show up in your service center. download and install any product updates.
you should be all set.
you didn't say what product it is but if it's a kontakt library NI should have added the product to your account and you should see some download links.