In short, nowhere. 3ivx is a codec suite which means you need a 3rd party application (eg. a media player like QuickTime or iMovie if you want to encode your videos) in order to use it. For instance, if you were to try to play an AVI encoded in MPEG4 Advanced Simple Profile in QuickTime, you would get a black video screen as QuickTime does not have the appropriate codec to decode it. Once 3ivx is installed, you should be able to see video. The 3ivx codec suite also allows you to compress video. If you have QuickTime pro, open any video, go to File > Export and choose 'Movie to 3ivx MPEG-4' in the export drop-down menu.
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