Minecraft – .minecraft folder doesn’t have the correct files

Point finder to the Hard drive the install is stored on (Macintosh HD is a common name) and type minecraft.jar into the search. Then right click minecraft.jar and click "Show enclosing folder" and you should be in ~/path/minecraft/bin

If you rename the .minecraft folder temporarily to say, .minecraft-old, then run the game, this will force it to redownload everything. Then, start a new (temporary) world, and exit. This should recreate the missing folder.

After you've done this, you can copy your old worlds from the .minecraft-old directory into the new .minecraft directory.