OracleXE and Designer connection problem

I'm a very newbie in Oracle. I've installed the latest OracleXE server and created a new user and some simple tables. Next, I installed in the same machine the Oracle Developer Suite, including Oracle Designer. But when I launch Designer, I can't pass the login screen as I do not know what to put in the "Connect string" field. I've tried with "127.0.0.1" which results in a ORA-12514 error (The SERVICE_NAME in the CONNECT_DATA was not found in the listener's tables.)

The contents of the relevant server configuration files at C:\oraclexe\app\oracle\product\10.2.0\server\NETWORK\ADMIN\ are as follows:

• LISTENER.ORA SID_LIST_LISTENER = (SID_LIST = (SID_DESC =
  (SID_NAME = PLSExtProc)
  (ORACLE_HOME = C:\oraclexe\app\oracle\product\10.2.0\server)
  (PROGRAM = extproc)
  ) (SID_DESC =
  (SID_NAME = CLRExtProc)
  (ORACLE_HOME = C:\oraclexe\app\oracle\product\10.2.0\server)
  (PROGRAM = extproc)
  ) )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC_FOR_XE))
(ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))
    )
  )

DEFAULT_SERVICE_LISTENER = (XE)

• SLQNET.ORA SQLNET.AUTHENTICATION_SERVICES = (NTS)
• TNSNAMES.ORA XE = (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521)) (CONNECT_DATA =
  (SERVER = DEDICATED)
  (SERVICE_NAME = XE)
  ) )

EXTPROC_CONNECTION_DATA =
  (DESCRIPTION =
    (ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC_FOR_XE))
    )
    (CONNECT_DATA =
(SID = PLSExtProc)
(PRESENTATION = RO)

    )
  )

ORACLR_CONNECTION_DATA =
  (DESCRIPTION =
    (ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC_FOR_XE))
    )
    (CONNECT_DATA =
(SID = CLRExtProc)
(PRESENTATION = RO)

    )
  )

And the content of the file tnsnames.ora where the Developer Suite is installed (C:\DevSuiteHome_1\NETWORK\ADMIN) is:

• TNSNAMES.ORA EXTPROC_CONNECTION_DATA = (DESCRIPTION = (ADDRESS_LIST =
  (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))
  ) (CONNECT_DATA =
  (SERVICE_NAME = PLSExtProc)
  ) )

What should I put in the "Connect String" field ???. Of course both the server and listener are running, and the Oracle web administrator interface works perfectly.

Thanks in advance,

-- 
Mik