What happens when you just install Flex Builder on the other machine and enter the serial number? I was not aware of Flex Builder having a mechanism that tracks activations like the Creative Suite products.
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Flex Development and Support Services
It will ask to register and asl for the serial number of licence.
I think we have licence which is restricted to for 3 users.
Personally when it was installed on machine they just uninstalled it on the old machine, then installed it on mine.
that was it