Hey all,
This worked!
You gotta do it slightly different on mac because of ZIP uselessness.
Rename "modpack.jar" to "modpack.zip", unzip it, open the new modpack folder and inject the optifine files (copy/paste/add/replace), then select all of the files inside the folder (cmd+a), right click and "Compress xxx items.." This will make an "Archive.zip" file inside the modpack folder. Move this zip file back up one level out of the modpack folder into the "bin" folder, rename it to "modpack.jar" and you're good to go!