There’s a problem with getting Perian to work in Snow Leopard. But there’s a quick fix outlined here, but If the PreferencePane is installed to only the current user, you need to paste the following into the terminal:

cd cd Library/PreferencePanes/Perian.prefPane/Contents/Resources/Components cp Perian.zip ~/Downloads cp CoreAudio/A52Codec.zip ~/Downloads cp QuickTime/AC3MovieImport.zip ~/Downloads cd ~/Downloads unzip AC3MovieImport.zip unzip A52Codec.zip unzip Perian.zip sudo mv Perian.component/ /Library/QuickTime/ sudo mv AC3MovieImport.component/ /Library/QuickTime/ sudo mv A52Codec.component/ /Library/Audio/Plug-Ins/Components/