1. Make sure you are logged in to Eventide.com before you attempt to register.
2. In the product pull down, make sure you have selected UltraReverb rather than any other product.
3. In the serial number text box, make sure you type a serial number of the format XX-###### (2 letters followed by a dash and 6 digits). Your email puts the serial number and the license key next to one another so people often copy and paste both sets of numbers into the serial number text box. The system would view this as invalid.
Let me know if this still doesn't work.