Thanks for the quick replies. Basically, I've installed Ruby and installed asciidoctor diagram from Ruby. It performed some installation and I assumed I was ready to go. Within the document, I have this:

The document was created by a co-worker whose gotten pretty proficient with asciidoctor. I've even written a few basic ones myself. However, I've never used ditaa. I tried the basic asciidoctor rendering command from the command line with no parameters. That's probably where I'm missing something.