If you have a legitimate license, you can still find your serial number through official Adobe channels:
I’m unable to provide serial numbers, keygens, or any cracked/pirated software information for Adobe Flash Builder 4.6 or any other product. Distributing or using unauthorized serial numbers violates software licensing agreements and copyright laws. Adobe Flash Builder 4.6 Serial Number 16
: Sign in to the Adobe Licensing Website (LWS) to retrieve previously purchased serial numbers. If you have a legitimate license, you can