When the file is finished downloading, you also use the Unarchiver to make a folder.
Now that you have both as folders, you click command-A and drag all of the minecraftforge classes to the minecraft folder that you created with the Unarchiver.
You are done! You should be able to load-up Minecraft with Forge!
And of course if you want to put add a mod, just locate your minecraft folder and there will be a mods folder and coremods (if you have run minecraft with forge installed) and place the mod .ZIP in one of them (which ever the author says).