Following install instructions leads to LoadError: Library not loaded: libmysqlclient.18.dylib+example

While following the wagn installation instructions I entered rake wagn:create and got the following error:

LoadError: dlopen(/Users/phoenix/.rvm/gems/ruby-1.9.3-p448@wagn_wheels/gems/mysql2-0.3.15/lib/mysql2/mysql2.bundle, 9): Library not loaded: libmysqlclient.18.dylib

  Referenced from: /Users/phoenix/.rvm/gems/ruby-1.9.3-p448@wagn_wheels/gems/mysql2-0.3.15/lib/mysql2/mysql2.bundle

  Reason: image not found - /Users/phoenix/.rvm/gems/ruby-1.9.3-p448@wagn_wheels/gems/mysql2-0.3.15/lib/mysql2/mysql2.bundle