It seems that you are having some issues in order to enter the Slingbox AV serial number when trying to take advantage of the Slingbox Upgrade Program.
The following thread will provide you with the instructions to follow in order to take advantage of the Slingbox Upgrade Program:
Upgrade Voucher problem
Hope this helps!
Since you own a Slingbox AV you are eligible to get the $50 discount on a purchase of a Slingbox PRO-HD or a Slingbox SOLO, if you are currently located in United States or Canada.
Try using "X20" only as the serial number, that should work.
Let us know if it worked.