Currently Being ModeratedNov 26, 2012 10:01 AM (in response to Marijosevm)
When you buy from the App store, Apple send out an email containing a purchase receipt and licence containing the serial number. Look for this email.
If you have deleted it, try logging into your app store account in previous purchases and see if your purcase is listed. Apple keep data for upto 18 months only. If either of these ways does not work you will need to buy a licence again.