If you were able to uncomment the line for the extension and do not see the change then you are probably missing the dll file in your extensions directory.
Did you use a php installer or did you install xamp or something like that? I know that with some of the preconfigured installers the extensions are not loaded by default. In my installation I have php_mysql.dll in the extensions directory and libmysql.dll in the php folder.
If you are missing these you can get them from the zip package on php.net. If you cannot get MySQL enabled after doing these things you may need to install it differently.
Have you been able to confirm that your MySQL is installed correctly?