I understand this entry of the FAQ:
software tools commonly used by programmers
in the Stack Overflow context as programming tools, i.e. IDEs, compilers, editors, build tools, source control management systems, etc.
I don't see Visio as fitting, unless you're dealing with macros/automation. (Trying to generate code stubs/schema from your diagrams would be on topic from my point of view.)
The question you have is an "end user" question. The question itself (the problem you have getting a particular shape/diagram symbol) is hardly programming related. The fact that an ERD is involved doesn't quite cut it, in my opinion.
That said, your question is clear, well written and presented, and I believe in scope for SuperUser. You could ask for it to be migrated there.