Certain mods for minecraft work only with previous ModLoaders. v4 mods will not work with v5 ModLoader and so forth. This will only work until the next update which will break the mod.
If you have modded before and not used ModLoader, it is in your best interest to start with a fresh minecraft.jar.
Okay if you are going to mod you must agree with this disclaimer.
Disclaimer: DO NOT SKIP THIS! Modding alters minecraft.jar and may cause SMP not to work with this mod installed. Mods can add new content and it is a bad idea to load old worlds that you played in before you installed this mod. We are not responsible for any accidental damage to your Minecraft copy or computer. *** YOU MUST BACK UP YOUR WORLDS! ****
For Mac users (only ModLoader) :
Sorry I don’t understand much of this, but is this what it says.
1. Go to Applications>Utilities and open terminal.
2. Type in the following, line by line:
jar xf ~/Library/Application\ Support/minecraft/bin/minecraft.jar
3. Outside of terminal, copy all the files and folders into the mctmp directory.
4. Back inside terminal, type in the following:
jar uf ~/Library/Application\ Support/minecraft/bin/minecraft.jar ./
rm -rf mctmp
5. Run and Enjoy!