Alright, I had this problem when I downloaded the hacks the the minecraft.jar I had an error message that said:

Could not find main class: Main

It also had a bit more, which doesn't really matter.

Firstly download this, the latest version of Java.

Now download this

Now download the hacks Assuming of course you're wanting the flying etc.

• Create a new folder somewhere (Either My Documents or Desktop)
• Name it something like Minecraft Hacks
• Extract the hacks to this folder (Assuming you're using WinRar)
• Double click the lib folder
• If you're using Mozilla, right click the downloaded client, and click "Open containing folder". You should see "minecraft" which is a .jar file.
• Copy & paste this to your lib folder (the one you just double clicked)
• Hit the back button on your files
• Right click the the run.bat file, and click Run as Administrator

This should fix the issue some people are having.
Hope this helped.