Want to install multiple Minecraft 1.11 mods at once on a Mac? 
This video will show you everything you need to do in order to install Forge for Minecraft 1.11 on your Mac. After this, you'll have everything you need to setup Minecraft Forge 1.11 on a Mac.
Simple problem to fix. Either you’re using an outdated version of forge, or minecraft, or you’re deleting meta-inf after you install forge.