Technic Solder Helper is a tool that helps you pack modpack files in the correct format.
Supports Technic and FTB packs. AT Launcher are in the works.
A compiled version of this can be found at: http://zlepper.dk/solderhelper/publish.htm
For linux just download the installer.sh file and run it. It will install the program in your home directory, though it can be changed in the script.
For Mac I have no idea if this will work or not. I don't have a mac, so I can't test.