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Do you get an error message?
Is it a real serial number or is it an redemption code?
In case you have an redmeption code take a look here:
Is the serial sequence all numbers or does it have letters in it also?
If it has letter in it then that is a Redemption Code the you Redeem for a proper Serial "NUMBER". The serial number will be 24 Character long, 6 groups of 4 numbers and starting with 1057.
Look here to redeem your code for a real serial number.
Also WRITE it Down and save it, Save multiple copies of it, because once the redemption code is redeemed it CAN NOT be used again.
Try entering the correct number using copy/paste. Strange but this worked for a visitor with a similar problem recently.
How did you use the computer Copy and Paste function when the number is on a piece of physical Paper?
Type the 24 digits and dashes into a word processing or text document and use ctrl+c/cmd+c
In the activation dialog click in the first box and use ctrl+v/cmd+v