Hey Stooovie, they don't install like addons, you have to click the Blender icon in the top left of the screen, and in that dropdown you will see "Install Application Template", once you click on that you can install the zip.
This template is getting a little old in the tooth, and I haven't updated it nearly as much as I'd like to, I've had some reports that it's not working well with 2.9, so I'll have to do a bit of updating at some point. I started on a different project to do the same thing (unify Blender and Unreal controls), but I haven't had a chance to polish and release it.
HOWEVER, as part of the new approach, I'm trying to make more of the Blunder features available as separate addons, which you can check out here: https://github.com/SpectralVectors/
There's UnrealRightClick for Right Click Navigation, Unreal Objects for a set of new default objects scaled up to 0.01 unit scale, and a tool to translate Blender's Shader Nodes into Unreal's Material Blueprint Nodes.
Thanks for checking out the project!