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First it goes without saying that you have to have the Stuff Pack installed on your computer in order for its contents to show up in Sims 4 Studio (or other tools) - I know it might sound silly, but I've been surprised at the number of people I've seen asking why Stuff or Expansion Pack items don't show up in S4S or similar tool, when they don't own the Pack in question...!
The next thing is to make sure you have the latest version of S4S, because versions of it from before a particular Pack came out won't be able to recognise the new content.