Our Compositing Packs are sets of video clips and therefore they don't have any project files so you don't need to install them anywhere in particular (that's also why our mInstaller app is only downloading them). All you need to do is extract the downloaded archives and place folders containing extracted files wherever it's convenient for you.
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To use one of the pack's components, drag & drop it over your footage/template, change its position, scale, rotation, and blending mode if needed, adjust its opacity et voila!
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We also recommend creating a separate event in FCP for each compositing pack and dragging the pack's folders to that event afterward – this way you will be able to easily preview and use them at all times (if you are low on disk space, you might want to make FCP keep the original media in their original place – you can do that via Preferences > Import > "Leave files in place" option).