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DivinylsFan  Just checking, but once unpacked, is it a .Sims3Pack or a .package file?  You probably know what to do with .package files already, but I have to ask.

Aside from that, try it in a clean user folder and see whether it works.  If not, feel free to link the original and I'll try it.

There's no special process for installing or reinstalling non-Store content.  The encryption on Store stuff breaks at times, but that doesn't apply to third-party files.

I think it may be because have it in the wrong folder, in Downloads instead of in Mods, which is where it is in a whole intact old Sims 3 folder on my external HD.  I'll move it and try again :P Thanks, your 'you know what to do' thing triggered my realisation. I bet I put there when I was redownloading everything, after not having to for yonks.