Could you add another comment that explains the whole issue a bit more clearly, now that you completely understand it and have solved it? Writing for the next poor schleb who is banging their head against this ... and so that the rest of us know more about this slightly-arcane setup?
You can configure all of this options or less. In MY_DB_NAME you create an alias for the configuration; in "host" you enter the db's ip and in "port" the used port (usually 1433 for MS SQL). In "database", if you need, you enter the start database, and you can also add values for "user" and "password".
I've seen in various forums that the best working tds's version for MS SQL is 7.0, so it's recommended add the "tds version" value with "7.0" such in the example.
After, you can recall this connection configuration simply by the alias, like this /usr/local/freetds/bin/tsql -S MY_DB_NAME.
If you're missing the "user" and "password" values nevermind, you can always launch tsql in this way /usr/local/freetds/bin/tsql -S MY_DB_NAME -U my_user, and so on.
I hope it's helpful.