I think you need to add /usr/lib/libiodbc.a to your Xcode project. also you will want to look into sql.h and sqlext.h assuming you're connecting to database such as MySQL to see what's the API you will use to communicate thru ODBC. But the code should be purely C++, nothing InDesign specific, you can write it as a separate library or an InDesign Boss object.
You will also need to install ODBC driver that's probably would be supplied by your database vendor. So for MySQL, there is a driver you need to download from MySQL site and install, and configure thru OSX's ODBC Administrator tool.
Thanks!
lee