So this was one attempt at adding a mod to the new launcher - directory structure was like this: versions/ | --1.5.1-modloader --1.5.1-modloader.jar --1.5.1-modloader.json --1.5.1.jar --1.5.1-natives/ | ----*lib* ---ModLoader/ | -----*.class

1.5.1.json

{
  "id": "1.5.1",
  "time": "Apr 28, 2013 3:09:12 PM",
  "releaseTime": "Mar 20, 2013 12:00:00 PM",
  "type": "release",
  "processArguments": "legacy",
  "libraries": [
    {
  		"name" : "net.minecraft.src.ModLoader", 
  		// is this the directory the files are in?
  		"ModLoader": {
  		// is this  [platform] : [directory]-[platform]
        "linux": "ModLoader-linux",
        "windows": "ModLoader-windows",
        "osx": "ModLoader-osx"
      	},
      "extract": {
        "exclude": [
          "META-INF/"
        ]
      }
  	},
    {
      "name": "net.sf.jopt-simple:jopt-simple:4.4"
    },
    {
      "name": "org.ow2.asm:asm-all:4.1"
    },
    {
      "name": "org.lwjgl.lwjgl:lwjgl:2.9.0"
    },
    {
      "name": "org.lwjgl.lwjgl:lwjgl_util:2.9.0"
    },
    {
      "name": "net.java.jinput:jinput:2.0.5"
    },
    {
      "name": "net.java.jutils:jutils:1.0.0"
    },
    {
      "name": "org.lwjgl.lwjgl:lwjgl-platform:2.9.0",
// is this the directory the files are in?
      "natives": {
      // [platform] : [directory]-[platform]?
        "linux": "natives-linux",
        "windows": "natives-windows",
        "osx": "natives-osx"
      },
      "extract": {
        "exclude": [
          "META-INF/"
        ]
      }
    },
    {
      "name": "net.java.jinput:jinput-platform:2.0.5",
      "natives": {
        "linux": "natives-linux",
        "windows": "natives-windows",
        "osx": "natives-osx"
      },
      "extract": {
        "exclude": [
          "META-INF/"
        ]
      }
    }
  ],
  "mainClass": "net.minecraft.client.Minecraft"
}