This particular aspect is a bit complicated!
As you can read on this page: http://store.apple.com/uk/product/D2187Z/A/quicktime-mpeg-2-playback-component-f or-mac-os-x
On OS X Lion and later, MPEG-2 playback functionality is built into QuickTime X. The QuickTime MPEG-2 Playback Component is not required and cannot be installed.
From the MPEG Streamclip homepage
The installer of the MPEG-2 Playback Component may refuse to install the component in Lion. Apple states the component is unnecessary in Lion onwards, however MPEG Streamclip still needs it. See this:
http://support.apple.com/kb/HT3381
To install the component in Lion, please download MPEG Streamclip 1.9.3b7 beta above; inside the disk image you will find the Utility MPEG2 Component Lion: use it to install the MPEG-2 Playback Component in Lion. The original installer's disk image (QuickTimeMPEG2.dmg) is required.
The current versions of MPEG Streamclip cannot take advantage of the built-in MPEG-2 functionality of Lion. For MPEG-2 files you still need to install the QuickTime MPEG-2 Playback Component, which is not preinstalled in Lion. You don't have to install QuickTime 7.