I've FlashEff2 installed on CS5 without any problem. Tried to install it on Flash CS5.5 using the Adobe Extension Manager CS 5.5, it says it is installed. But when I open Flash CS5.5 it doesn't show the components, nor the FlashEff2 windows.
The component doesn't show up because you didn't open the extension manager directly from Flash. If you will open the extension manager by going to help->manage extensions and then install the component, you will see that the component is there. Just follow exactly the same steps you see in the video tutorial from below.