Cannot enter serial number(s) in the Manage Flex Builder Licenses window (Flex Builder 3)
To resolve this issue,find and rename or delete the proper license.properties file from your system.
In Windows 2000 and Windows XP the license.properties file is located in:
..\Documents and Settings\All Users\Application Data\Adobe\Flex
Note: If the file is not under "All Users," it may be under a specific user account.
In Windows Vista, the license.properties file is located in:
In Mac OS/X, the license.properties file is located in:
Once you have renamed or deleted this file, you will be able to enter your new serial number the next time you start Flex Builder.