In think its cos recent versions of my sql have changed their path from lib/mysql to just lib. A mac friend suggests you open a terminal and type:
sudo install_name_tool -change /usr/local/mysql/lib/mysql/libmysqlclient.15.dylib \ /usr/local/mysql/lib/libmysqlclient.15.dylib /usr/local/lib/ruby/gems/1.8/gems/mysql-2.7/lib/mysql.bundle
I'm a linux user so I have no idea if this will work or not.
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