To work around the problem, I went to the Administration Console, and added the root domain (even though we are not managing this).
Once that entry was present, the web interface customization tool started without error. I was able to add the customattribute, and get it to appear in the web interface.
Still not sure this is the answer to the challenge I have been presented with. For example, even though we can set adding a user to a group to be routed for workflow approval, our support staff uses another system to explain "why" someone needed to be added to that group. They want ADAxes to be able to pass the ticket number from this system along with the workflow so that the approving person can cross reference this as they approve the ADAxes workflow.
Thanks