#
# HS init parameters
#
HS_FDS_CONNECT_INFO = MSSQL
HS_FDS_TRACE_LEVEL=OFF
HS_LANGUAGE=AMERICAN_AMERICA.WE8ISO8859P1
HS_FDS_FETCH_ROWS=1
HS_FDS_PROC_IS_FUNC=TRUE
HS_KEEP_REMOTE_COLUMN_SIZE=LOCAL
HS_NLS_LENGTH_SEMANTICS=CHAR

#
# Environment variables required for the non-Oracle system
#
#set <envvar>=<value>
5.2	Changes for file listener.ora 

The listener.ora file must be changed.  The part shown in bold must be added.

# listener.ora Network Configuration File: C:\Oracle\product\11.2.0\dbhome_1\network\admin\listener.ora
# Generated by Oracle configuration tools.

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = CLRExtProc)
      (ORACLE_HOME = C:\Oracle\product\11.2.0\dbhome_1)
      (PROGRAM = extproc)
      (ENVS = "EXTPROC_DLLS=ONLY:C:\Oracle\product\11.2.0\dbhome_1\bin\oraclr11.dll")
    )
    (SID_DESC=
      (SID_NAME=MSSQL)
      (ORACLE_HOME=C:\Oracle\product\11.2.0\dbhome_1)
      (PROGRAM=dg4odbc)
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = MRA-NB)(PORT = 1521))
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
    )
  )

ADR_BASE_LISTENER = C:\Oracle

And check sqlnet.ora at client side:

Should contain following line:
SQLNET.AUTHENTICATION_SERVICES= (NONE)
5.2.1	Listener restart

When changes are made to listener.ora-file, then the listener itself must be restarted.

To restart the listener, from command-prompt (via Run as Administrator):
lsnrctl stop
lsnrctl start

5.3	Changes for file tnsname.ora

The tnsnames.ora file must be changed to make it possible to use the ODBC-connection via an entry in this file.

MSSQL = 
   (DESCRIPTION =     
    (ADDRESS_LIST =       
     (ADDRESS = (PROTOCOL = tcp)(host= MRA-NB)(port=1521)) 
    )     
    (CONNECT_DATA = (SID = MSSQL))
   (HS=OK)
  )

Test connection via a command-prompt and by entering the command:

TNSPING MSSQL