When I get this error, it invariably means that the Management Server service hasn't been started, or has shut itself down because of some error, such as an inability to connect to the repository database. If you're on Windows, you can start it up by going into Start -> Programs -> Administrative Tools -> Services and starting the Windows service.
As the error message indicates, you might also be trying to connect to a host that doesn't have an Oracle Management server installed. This might be because of incorrect connection information.