Are you trying to open your Minecraft.jar and put ModLoader inside it?
If you are you need WinRar
http://www.rarlab.com/And You change skins Via minecraft.net.
Once you have WinRar Installed/ You might not have to install it i can't remember but anyway once it is set up.
Simply Right Click on your Minecraft.jar
Open with WinRar and drag the files from Modloader into the Jar and delete Meta Inf.
If for some reason you mess up you can Go to minecraft menu and click force update and it will refresh your minecraft when you login
Directing my Post to Person above ^ not the OP