Adding a promotional code

I need to set up a promotional code for several of my products, but can’t work out how to do it. I’ve looked at Inventory where it says there should be an option create a promotion but there’s no option on my drop-down. I’ve looked at the various options in the inventory against each item, but nothing there.This will be a special on-off code to last for one week, which will be promoted at a conference where I’m speaking. Can anyone help please?