use explorer to navigate to the tnsnames.ora file on the database server. Copy the contents of that file, and close it. Navigate to the tnsnames.ora file on the client (where SQL*PLUS is to be run), copy that tnsnames.ora to tnsnames.old, then open it, go to the bottom and paste the copied values. Save as tnsnames.ora.

connect username/password@service_name

Good luck!