If you installed MySQL using the official .dmg package provided by MySQL AB/Sun and are trying to install the native MySQL gem, you’ll need to pass a bunch of flags to the gem install process to get the thing to compile. Try this..
sudo gem install mysql — –with-mysql-config=/usr/local/mysql/bin/mysql_config
..if you still have issues (check your /opt/local/lib/ruby/gems/<ruby_ver>/gems/mysql-<gem_ver>/mkmf.log file), try this instead..
sudo gem install mysql — –with-mysql-dir=/usr/local/mysql –with-mysql-include=/usr/local/mysql/include/ –with-mysql-lib=/usr/local/mysql/lib/ –with-mysql-config=/usr/local/mysql/bin/mysql_config
Run `sudo gem install mysql` and definitely recheck the log file again as the gem install process sometimes reports false positives even if native components fail to install.