You first need to set up the TDS connection in /etc/freetds/freetds.conf
Then set up the DSN in /etc/odbc.ini (and possibly /etc/odbcinst.ini)
examples (Debian Whezy):
/etc/freetds/freetds.conf
[global]
text size = 64512
[<arbitrary label>]
host = <hostname or ip address>
port = 1433
tds version = 7.2
/etc/odbc.ini
[ODBC Data Sources]
[<dsn name>]
Servername = <label from freetds.conf>
Database = <database name>
Driver = FreeTDS
/etc/odbcinst.ini
[FreeTDS]
Description = TDS driver (Sybase/MS SQL)
Driver = libtdsodbc.so
Setup = libtdsS.so
CPTimeout =
CPReuse =