The latest release for Molly is 0.8. You are using an outdated version, which is why the examples in the documentation won’t work. You can either install the latest version of Molly or read an older version of the documentation (in the lower left corner of the docs you’ll find a dropdown menu to select the version).
Have a look at this related thread:
and read the relevant sections of the Pkg.jl manual, e.g.