It sounds as though you need to install
Perian in order to allow playback/conversion with QuickTime and thus MPEG Streamclip. Failing that, it's likely that
ffmpgX will handle conversion as it exists completely separately from the QuickTime Framework.
VisualHub provides a slightly less complicated interface but is not free.
In order for us to help you further, open the file in VLC and Get Info (command-I). Select the Advanced Information tab and report what info it gives about Stream 0 and Stream 1.
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