If you are trying to run your app with Rails and you got some errors when execute the rake db:migrate It is possible that you must to install the GCC. In Lion installation there is no gcc default. You can check GCC here: https://github.com/kennethreitz/osx-gcc-installer
Other possible error is the lack of a symbolic link:
( Library not loaded: libmysqlclient.18.dylib )
ln -s /usr/local/mysql/lib/libmysqlclient.18.dylib /usr/lib/libmysqlclient.18.dylib
With these changes you may run successful the mysql + rails in your app.
Remembering that is a good practice to reinstall your gem after these changes:
gem uninstall mysql2