I did find a solution and was able to successfully export a model to the reporting repository. Here are the steps:
the setting for "system data types directory" in "Preferences>Data modeler" was pointing to an old copy of defaultRDBMSsites.xml and types.xml. That copy of defaultRDBMSsites.xml did not include an entry for Oracle 12c. When I changed the pointer to point to the types directory that came with DM 4.0, the import worked.