I've purchased a product, but it's not showing up in mInstaller

If you've just purchased CineStudio or DesignStudio plan, you need mExtension - not mInstaller. mInstaller is only for legacy perpetual licenses purchased outside subscription offer.
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If you're trying to install a product purchased a one-time payment order, make sure that mInstaller is up-to-date - you can do that in mInstaller settings. Once it's updated, check if you can see your purchases.