I know it's too late to answer, but I've been fighting with that exact configuration last week and I can tell you what you have to do. You MUST use ODBC to access SQL Server ( from UNIX/LINUX, MAC or Window$ )

- In MAC, install macports and through this, freedts & iodbc ( perhaps you can use the one from OpenLink, but I've uninstalled it... )

You can configure odbinst.ini like this :

[SQL Server]
Description = FreeTDS Driver
Driver = <path_to_lib>/libtdsodbc.so
Setup = <path_to_lib>/libtdsodbc.so

and you can configure freetds.conf like this :

[global]
...

[SQLServer]
tds version = 8.0
client charset = UTF-8

For the connectionString, you must use one like this:

"DRIVER=FreeTDS;SERVER=<ip_or_server_name>;UID=<yo ur_uid>;PWD=<...>;SERVERNAME=SQLServer

Don't forget to set in ODBC's connection options "SQL_ATTR_ODBC_VERSION=SQL_OV_ODBC3"

Currently I've found that you must add this when you connect to SQL Server from MAC.
Perhaps last version of FreeTDS 0.82 needs this or perhaps it's my Qt's ODBC driver, I don't know...

..and all must work !!