That should be all you need to do. Make sure you tried closing ObsCore and rerunning it - I believe it only processes eops when it starts up.Sorry, I'm a little confused. I dowloaded a few plugins from here: https://edjp.colacube.net/observatory
They're eop files as explained in the OP. I've copied them to the Observatory plugins subfolder, but they they don't "work" as it were - Observatory doesn't install them. Do I need to rename them as zips then extract them?
Failing that - are you definitely running Observatory Core and not the older Observatory? If so, did you use the stand-alone version or the installer version? If you used the installer version, you can try closing ObsCore and simply double clicking an eop - it will install it and then open ObsCore. Also, if you haven't already, make sure you're running the latest version of ObsCore too.
They are simply renamed zips, so you can try manually extracting them - but it absolutely shouldn't be necessary.