I am assuming that you are running MystCraft. And if you are, you should know that MystCraft is 100% incompatible with the newest Forge versions. XCompwiz(I think that is his name) said that a fix is coming
The issue is that the stock archiving utility on Mac OS X 10.7.4 (and later or below) are only capable of handling .zip(as to my knowledge as a mac user) so you would have to go onto the App Store and download The Unarchiver, it is free and best of all supports unarchiving .jar files.
I think that it would be best if you told the creator of Millenaire about that. Since i think that there is little that Forge can do about this kind of issue
[/s]Can someone explain the methods to install mods? Because I am now really confused with the CoreMods folder, on should I install base editing mods into the jar or the CoreMods Folder