Flex/AMF3 recording requires at least two files:
  • flex-messaging-common.jar
  • flex-messaging-core.jar
Since SilkPerformer 2010 these files are located in:
  • C:\Program Files (x86)\silk\SilkPerformer x.x\ClassFiles\Adobe-Flex

These files provide the framework functionality for AMF3 applications. 
For older versions of SilkPerformer they can be downloaded from the attachments section below (when downloaded change the extension from .zip to .jar).

Please note your application may require other jar files for the recorder to transform the traffic correctly. You should add these files through the Model Script Dialog. These files are provided by Adobe at the Flex Developer Centre or alternately can be obtained from your Application administrators.