Don't Use This Any More. It's not supported, it's ages old, it's not meant for the new versions of Minecraft.
With this utility, you can install any mod into your Minecraft jar file.
Just start up the java application, and choose your downloaded mod. And press install
It will then check to see if the mod needs Modloader.
If it does, but you don't have it installed, it will tell you, and ask if you wish to continue. Then, it checks to see if the mod can be copied straight into the mods folder in your .minecraft folder. If so, it will do that, otherwise it will copy all of the files into the mineraft.jar and re-pack it.
If your mod does not require Modloader, it will just copy the files into your minecraft.jar.
If you select the box labeled "check the box labeled Check If Classes Are Within A Sub-directory" then it will find the closest class file, and copy the files from within the folder.(E.G: Inside a zip folder you have three folders: Source, Classes and Config. Without the check box checked, it will just copy the files into the minecraft.jar. If it is selected, it will then search through the zip to find the closest class file, and then install from the directory onwards(If you don't understand what I'm talking about, just leave it checked, and the install should work most of the time.))
I'm not sure I understood that last part- will we have to change the way we pack our mods for them to
work with this installer?
No not at all. What it does, is it scans the zip archive for a class file and then copies all of the files from there into the minecraft.jar. So for example, in Zombe's Mod pack he has uploaded a zip file like this:
If you just copied all of these files into the minecraft.jar then the mod would not work. So what my tool does, is it searches the folders for a class file, and then copies the files from that folder straight into the minecraft.jar. So instead of copying the classes folder, it copies the contents of the classes folder.
If that still doesn't make sense(which would be because of my horrible explaining skills), then don't worry. Just leave the box checked. It will improve the chances of a successful install.
So no, you don't have to change how you pack your mods. This tool should automatically adapt to the format of the archive.
So I used this to install your "My People" mod for Minecraft 1.2.3 and when I first loaded up the game after using it, it crashed. I thought to myself "Okay, stop being lazy, just do it manually." I went to the launcher, force updated and it would download all of the files in the "bin" folder of your .minecraft file, but then wouldnt come up with the mojang symbol. I tried completely deleting the .minecraft file, deleted the Minecraft.exe launcher, redownloaded the launcher and force updated. Still hasn't let me in.