With 1.6 Minecraft switched to a new launcher, one with lots of new features and options. It's useful, but it completely overhauls the way Minecraft handles .jar files. While it allows for easier management of multiple .jar files and lots of nice cool stuff, this switch does render your previous modding routines outdated. A new method is required, and you can read all about it right here…
Ok so you're trying to put all the files into the minecraft.jar, but the new launcher doesn't even use it and will just redownload if you mess with any of it's precious version .jars. You're confused. Don't worry, I was too. Luckily, even though it's a new process, it's simpler than the previous one, so let's get right to it.
Step 1: Download the new version of Forge.
Luckily for all of us, the developers behind MC Forge, the widely adopted mod API, made an installer version of Forge that will do a lot of the heavylifting for you. Get the “installer version” of Forge for your current version of Minecraft.
Step 2: Install Forge using the installer.
You'll download a .jar file. Double click it and it'll open a simple installer. Just click OK and let it do the magic.
Step 3: Select the Forge version of Minecraft in the new launcher.
Open the Minecraft launcher (the new one) and go to “Edit Profile”. There select the appropriate Forge version. It will most likely be called something like the one in the picture. Save Profile and Forge is ready to go!
Step 4: Forge is ready. Install your mods!
Now that you've got Forge installed, you're ready to install most of the mods out there by just downloading their .zip file and putting it into the “mods” folder like you would before. There may be mods that don't work through Forge, but these will have their own detailed instructions available with the download.
That's it. It's that simple! You can now enjoy all the new and old mods for 1.6.2 and above.
Let us know if you have any problems in the comment section below. We will respond ASAP and try to help you resolve it!