If you copy paste (which some people will definitely do instead of going to the GH repo) the two code samples from:
and infer file paths to create from the text on those pages, you end up with
tut_hello_cube.rb
tut_hello_cube/main.rb
That meshes poorly with line 8 in the loader script there, which references the directory ex_hello_cube:
ex = SketchupExtension.new('Hello Cube', 'ex_hello_cube/main')
Presumably this is related to #2, but seems more immediately problematic because the live example is broken as written.