Michael P. Ready
How do I specify which tnsnames.ora file to use?
that's not an application specific request, but something you need to tell your Oracle Client software.
The most straightforward way to do so is to define the environment variable TNS_ADMIN which contains the _PATH_ where the TNSNAMES.ORA resides.
E.g. your TNSNAMES.ORA is located in C:\ORACLE\NETWORK\ADMIN, then you would use the System Control Panel to define a environment variable called TNS_ADMIN and set the value to C:\ORACLE\NETWORK\ADMIN