After, to link the mysql-simple package I did an export LD_LIBRARY_PATH=/usr/local/mysql-5.6.11-osx10.7-x86_64/lib. Thus, the library libmysqlclient.18.dylib was found.
This is usually a bad idea, and what you're seeing is the least of the things that can go wrong.
The correct fix for all of this is not to use the environment settings, but to build the mysql binding with
so it actually finds and records the location of the library instead of using an unsafe override that you have to reproduce at runtime (doing that for anything launched from the GUI is difficult, and in some OS X releases impossible, and is always risky).