Magnolia 5.3 reached end of life on June 30, 2017. This branch is no longer supported, see End-of-life policy.
Now you need the license key that was sent to your email address. Copy only the key string from the message. Use the same email address you used when registering.
Before you paste the license key make sure that it is a continuous string of characters with no spaces or line breaks. If you have any problems you can paste the key to a plain text editor to strip out any formatting.