Can anybody help me with modding in minecraft?

Ok, I have a mac, an old mac that runs version 10.5.8. I want to install modloader, so I follow directions on youtube and turn minecraft.jar into a folder, which i renamed minecraft.jar. I installed it, and all the mods, and I ALSO deleted META-INF. However, when I start, I get a black screen. From what I figured out, I think that the minecraft client doesn't know that there is a minecraft.jar because it is in a folder form... or not? Anyways, I did everything, I even tried terminal, which didn't work at all. Can somebody please help me! I really want to play the game with mods!