Jeff M.

I think that I see the problem.

Instead of

[shortcode arg="hello_world"]

I should be doing this

[hello_world arg="Joe"]

Hi, Marcel:

I am using the default shortcode that was included with your plug-in.

extract(shortcode_atts(array('arg' => 'default'), $atts));
echo "Hello world!" . PHP_EOL;
echo "Arg=" . $arg . PHP_EOL;
echo "Content=" . $content . PHP_EOL;

If you check the site, I have examples executing (or attempting to execute) the code in both posts and pages.