I received a very rapid and accurate answer from Amy at Mathworks after reporting this issue.
Version 9.72 of the API does not support 64 bit and downloading to an earlier version fixed the issue. It did not however work for me on Windows 10 though others may have gotten it to work.
The immediate fix is to use the beta version of IB API 9.73 which is supported by Matlab 2018a per-release. I uninstalled my previous API and then 9.73, then Matlab 2018a per-release and it does work now. This is a quick and easy fix and I would like to commend the Mathworks team and Amy on a great product and rapid support.