View Single Post
The plus button is only active if you are in the Inbox or the Contexts or Projects folder. It is used to add new projects, folders, and yes, new actions.

It is grayed out in the home screen as well as the Due Soon, Overdue and Flagged views because it doesn't apply in those views.

Frankly, I find the easiest and fastest way to add a new action from the home screen of OmniFocus is to hit the Inbox icon at the bottom and type in the action. Usually, I don't even worry about putting it into a context or project until later in the day when I'm clearing out my inboxes. The goal is to get it out of my head and into my trusted system.