You should be able to find it under the SEO app after activating it.
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SEO Boost App
Are you referring to "Meta Keywords" and "Meta Description" available in General Settings? These two settings are not a part of the SEO Boost app -- they are available by default.
Meta keywords36 - A set of keywords to describe what the community is all about.
Meta description14 - This is useful to give a brief summary of the community. It might get featured on the Search Engine Result Page and impact the click-through rate.