Go to Users>(your name)>Library>Application Support>minecraft>versions>(minecraft version you're modding). If you can't see the Library folder, look up how to unhide it. Now ctrl-click the .jar file you see and compress it. Then download and install Zipeg (
http://www.zipeg.com) and ctrl-click the .zip file you made in the mentioned directory and Open With Zipeg. Click the "Extract" button near the top of your Zipeg window, then look on your desktop and open the new folder.
Follow the instructions from there. When you're finished, compress the modded folder, rename it as a jar, and replace the old .jar file.
Hope it works for you.
