When I go to view the extensions (Customize > Extensions) in ArcMap I see the license is listed as not available. Why ?
WaterGEMS, SewerGEMS, and HAMMER, which all integrated with ArcMap are not extensions in ArcMap. They are fully built into the ArcMap interface through the API. As long as you have your OpenFlows product license is set up in either the Bentley Licensing Tool or the Municipal License Administrator, the product license will work.
WaterGEMS for ArcMap and WaterGEMS for ArcGIS Pro FAQs
SewerGEMS for ArcMap and SewerGEMS for ArcGIS Pro FAQs
What level of ArcGIS license is required for ArcMap integration?