If you want direct access to the database without defining a TNSNAMES.ORA you have to be on the same machine as the database is installed. Just try the username/pwwithout the connect-string ("sqlplus system/manager" instead of "sqlplus system/manager@testdb".
If you get an ORA-12560 TNS:protocol adapter error, the ORACLE_SID environment variable does not seem to be set set it with "SET ORACLE_SID=yourdbsid" in Windows or "export ORACLE_SID=yourdbsid" on Linux/Unix. Hope it helps... 