I had this issue and it took me for a while to figure out how to fix that.
My case is slightly different. My MySQL server is of version 5.1.x. And somehow I upgraded my MySQL-python from 1.2.3 to 1.2.5. And I kept getting this issue since then event I added the following soft link.
libmysqlclient.18.dylib -> /usr/local/mysql/lib/libmysqlclient.18.dylib
It turns out that for MySQL 5.1.x there is no libmysqlclient.18.dylib, but only libmysqlclient.16.dylib. You can fix this issue either by downgrade your MySQL-python to 1.2.3 or upgrade your MySQL server to 5.6.x (I haven't tried 5.5.x.)
I downgraded the library to 1.2.3 since upgrading MySQL is not an option for me.