HEC-GeoRAS is an add on module in ArcGIS. you have to create layer from Hec-GeoRAS and exported to HEC-RAS. Analysis are carried out in HEC-RAS then export to Hec-GeoRAS. It not an automatic module.
HEC-GEORAS is an extension that works under ArcGIS environment, simply execute the setup of HEC-GEORAS, after you must open ArcGIS, you click tools, customize and then after you check the box where HEC-GEORAS located. after all is well. good luck.