You need to download a program such as 7zip what i use. then you need to right click on your minecraft.jar file open with, browse for program, look for 7zip file manager. use that program to open your minecraft.jar. It will either be in your program files or program files (x86) depending what you download.
A suggestion would be to use MagicLauncher to install your mods, but for certain ones you will need to open your minecraft.jar, if this is the case, I have no suggestions other then 7zip. I hope this was helpfull
It is happening to me as well and nothing I do will fix it. No mods no texture packs no worlds nothing it is a completly untouched new one but I still get that error. And it is really starting to make me really mad. What did they do to minecraft to make it do this?
so what ur saying is(this happened to me too) that u download 7zip and use it in ur little chose default programs and select that and run it?(oh and if that is what u do ive tried and for some reason i download it and when i choose which programs to run it from it doesnt say 7 zip any help here?
Same thing happened with me, but I found out how to fix it. For me all I had to do was go into my modded jar and go into META-INF. Once there, delete everything except Manifest.mf. Hope this helps!