ok, your modpack does NOT have a modpack.jar, it has a moded Minecraft.jar.
that is bad. you put ALL jar mods into the modpack.jar, as the Minecraft.jar is over righten with a default one every time the modpack is launched.
{EDIT}
if you rename the Minecraft.jar you put in your modpack's download to modpack.jar it works fine. so, do that.