you're using the cs5.5 serial number when prompted for the 5.5 version's serial number or when prompted for the 6.0 version's serial number?
if cs5.5 is already installed, you won't be prompted for the 5.5 serial.
if you're certain you're entering the correct serial at the correct prompt and still having a problem, contact adobe support: www.adobe.com/support/chat/ivrchat.html