It's not necessarily hard to, it's just that you don't know how to mod it.
All actions that are described after this line of text happen in the .minecraft folder.
Step 1: Open your .minecraft folder. If you're using Windows 7, open Windows Explorer (the folder icon) and highlight the text in the directory bar. Replace that text with %appdata%. You should be directed to C:\Users\YourUsername\AppData\Roaming. Once in this folder, find the ".minecraft" folder and open it.
Step 2: Find the "versions" folder located inside your .minecraft folder. You can think of this folder as the new "bin." Inside this folder, you will find the versions of Minecraft you have opened using the new launcher. If you're modding 1.6 and don't see a "1.6.1" folder, try opening it in the new launcher so it can install.
Step 3: Find whatever version of Minecraft you're modding by looking for the version name as a folder. So, if it's 1.6.1, there'd be a folder named "1.6.1." Copy this folder (highlight it and press CTRL + C in Windows) then paste it (CTRL + V in Windows) in the "versions" folder.
Step 4: Rename the folder that you pasted to whatever profile name you're going to set it as. Then, rename all the files (except for those in a natives folder) to the same thing. If one of the files is a .json file, open it up (preferably in Notepad++). Change the text that comes after "id": in quotes to the profile name. (Example: "id": "1.6.1 Modded",)
Step 5: Save the .json file (if edited), open the .jar using any archiving program, delete META-INF, and place the mod files into the .jar file (otherwise known as standard modding procedure).
Step 6: Open the new launcher, create a new profile, and use the drop down "Use version:" menu to select your modded version of Minecraft. It should work fine. If it doesn't, alert me of it.
