Make sure you have a full version of Kontakt 5 and not the free player version.
You do not need one. Again, the issue stems from you version of Kontakt 5. Please refer to the previous question.
When you unzip your product, do not tamper with the folders and files. If you have, log in to your customer account and re-download your product.
Make sure you have the latest version of Serum (minimum v1.215).
All purchases need to be linked to a working email address. If you think you entered your email wrong, contact us. If you have used an Apple address such as @me.com or @icloud.com, please contact us using another address. Unfortunately, we have been unable to reach these customers in many instances. We are currently looking at the problem.
Create a customer account. If that doesn't reset your downloads, contact us.
All purchases are fully refundable. Prior to buying, make sure you understand how Kontakt 5 and Serum products work, and that your software meets the minimum requirements.