What do I do if I don't have a META-INF folder or minecraft.jar?

Solution 1:

In 1.5.2 and earlier:

minecraft.jar is located in %appdata%\.minecraft\bin, and can be opened using many archive readers (such as 7zip, Jzip, WinRAR, etc.) as Schism mentioned. META-INF should be contained within this .jar. However, if META-INF is not in the .jar, you shouldn't need to worry about it, as the addition of mods requires it to be removed anyways. If you have installed mods in the past, you should have removed it then. It can also be removed by third-party tools such as MCPatcher.

In 1.6 and later:

There is no minecraft.jar anymore since the launcher has moved to a version-based system. The equivalent to minecraft.jar is now something like:

%appdata%\.minecraft\versions\<x.y.z>\<x.y.z>.jar

... where x.y.z is the version of Minecraft you are running and trying to mod. Modding Minecraft now is the same as before, except you're changing that x.y.z.jar file instead of minecraft.jar: open that x.y.z.jar file, place the mod's files as usual, and delete the META-INF folder before saving x.y.z.jar.