Oracle Database Discussions

Hello everyone,

I have installed both oracle 10g and oracle desktop discoverer successfully. My initial screen of desktop discoverer which looks like this:
http://i56.tinypic.com/nn70xj.jpg

and i have entered the following details:

username: system
password: sairam
connect: mydb

But i get the following error:

unable to connect to*: system@mydb*

failed to connect to database - ORA - *12154: TNS: could not resolve service name*


CONTENTS OF TNSNAMES.ORA

# tnsnames.ora Network Configuration File: D:\oracle\product\10.2.0\db_1\network\admin\tnsnames.ora
# Generated by Oracle configuration tools.

MYDB =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = OMSAIRAM-8F47AE)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = mydb)
)
)

EXTPROC_CONNECTION_DATA =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1))
)
(CONNECT_DATA =
(SID = PLSExtProc)
(PRESENTATION = RO)
)
)

Please help.

Thanks & Regards

Lucky