Put this in a 1.8.3 instance patches folder to get M3L working

com.cuchazinteractive.m3l.json

{
    "+libraries": [
		{
		"name": "cuchaz:m3l:1.8.3-0.2b",
		"url": "http://maven.cuchazinteractive.com/",
		"insert": "prepend"
		},
		{
		"name": "net.minecraft:launchwrapper:1.8",
		"insert": "prepend"
		},
		{
		"name": "cuchaz:m3l-forge-api:1.8.3-0.2b-1334",
		"url": "http://maven.cuchazinteractive.com/",
		"insert": "prepend"
		},
		{
		"name": "cuchaz:enigma-lib:0.10.4b",
		"url": "http://maven.cuchazinteractive.com/",
		"insert": "prepend"
		},
		{
		"name": "org.apache.commons:commons-lang3:3.1",
		"url": "http://repo1.maven.org/maven2/",
		"insert": "prepend"
		},
		{
		"name": "org.slf4j:slf4j-api:1.7.10",
		"url": "http://repo1.maven.org/maven2/",
		"insert": "prepend"
		},
		{
		"name": "ch.qos.logback:logback-classic:1.1.2",
		"url": "http://repo1.maven.org/maven2/",
		"insert": "prepend"
		},
		{
		"name": "ch.qos.logback:logback-core:1.1.2",
		"url": "http://repo1.maven.org/maven2/",
		"insert": "prepend"
		},
		{
		"name": "org.javassist:javassist:3.19.0-GA",
		"url": "http://repo1.maven.org/maven2/",
		"insert": "prepend"
		}
    ],
    "+tweakers": [
        "cuchaz.m3l.tweaker.TweakerClient"
    ],
    "fileId": "com.cuchazinteractive.m3l",
    "mainClass": "net.minecraft.launchwrapper.Launch",
    "mcVersion": "1.8.3",
    "name": "M3L",
    "order": 6,
    "version": "v0.2 beta"
}