Whats goin on here. First Clarify me.

Are you trying SQL*Plus from Server Binaries Or Client Binaries. It works diffrently.

For client Binaries, You will use tnsnames.ora and make sure that you have your net service name defined there in order to login.

For server binaries, If you have listener service running and if you set the sid name and try to login it should work. If server manager works after setting ORACLE_SID=xyz, SQL*Plus should also work.

Check for tnsnames.ora and listener service whichever appropriate for your situation and see if it works, otherwise post back your tnnames.ora and listener.ora where you have entry for the SID you are trying to connect.

good luck