That you are using administrator account doesn't mean that all applications you run have administrative privileges. To run Extension Manager with administrative privilege, you have to right click "Adobe Extension Manager CS5.exe" in Windows Explorer and select "Run as administrator" from the context menu. After you input the administrator password, Extension Manager will have the right privileges.
Since Extension Manager CS5.5, you can run Extension Manager as normal, the UAC dialog will pop up when administrative privileges are required so that you can continue the operation after accepting UAC prompt, no need to restart Extension Manager as administrator.
