Did you have it installed in your Snow Leopard (which you used to install Lion)? Can you copy the old one (not the installer, but the actual plugin) from the library folder to the same location? Other than that, I'm out of ideas except to give Apple a call or schedule an appointment at your nearest Genius Bar.
I've also had trouble installing the MPEG-2 component, but while checking my two Lion installs (one new, one upgrade from 10.6), I see that it already appears to be installed at /Library/QuickTime/AppleMPEG2Codec.component. I don't have an MPEG-2 file to play right at the moment, but I think this means that Apple has pre-installed the component with all Lion installations.
Can anyone else confirm this?