Minecraft Forums

The new launcher puts things in a different location.
The main .jar file now lives in .minecraft/versions/<version>/<version>.jar where <version> is whichever minecraft version you want.

For example the latest snapshot is at:
.minecraft/versions/13w17a/13w17a.jar
(Note there is no minecraft.jar, it's now named to reflect the version.)

1.5.1 should be at:
.minecraft/versions/1_5_1/1_5_1.jar

I think if you want your current modded MC, all you would need to do is just copy your minecraft.jar to the 1.5.1 location and rename it. Dunno though as I've not tried that myself.

Quote from OutrageousBunny

Curse, if you put a mod in the files the launcher automatically replaces your file with a new downloaded one because it thinks your file is up to date... So do you know how to fix this?

Copied from a similar thread

Quote from nerd-boy

You can "trick" the launcher to launch a modified jar you have to

• Copy the 1.5.1 folder (call it "1.5.1-Mod")
• Rename the jar from "1.5.1.jar" to "1.5.1-Mod.jar"
• Rename the natives folder from "1.5.1-natives" to "1.5.1-Mod-natives"
• Rename the json from "1.5.1.json" to "1.5.1-Mod.json"
• Open the .json file and edit the ID line so it has the value "1.5.1-Mod"
• Modify the .jar as you would normally

After this it should create a new option with 1.5.1-Mod that should be a modded minecraft

Just remember the new launcher is still being worked on and is not ready for prime time yet