In my testing, it appeared that the extension did not load when I had only CairoMakie and SomePackage installed in the main environment. The dependency of the extension is Makie, which is also always a dependency of CairoMakie (and GLMakie, etc.). But the users usually don’t install Makie directly. So I wonder if this is on purpose, or if I configured something incorrectly, etc.
You can clone GitHub - jkrumbiegel/MakiePkgExtTest and activate the outer environment. It currently has both CairoMakie and Makie installed. In that mode, I can access the extension. If I rm Makie, I can’t, even though it is still installed via CairoMakie.