Couple of things to do first:

Check whether the OMS has been started if not start it.

Then check your tnsnames.ora file. It is the service name that you have to give, which inturn would be your sid_name

Then the username is sysman/oem_temp

The node that your OEM connects to should have the OMS installed. If not I would suggest you to go ahead and install as terry said.

the host name should be the full path
example: hello.oracle.com

And then try. If you still have problem, let us know.

Sam