Occupation is already a fact type so you need not add it.
I think the problem is that you're clicking the downward pointing triangle on the right side of the Add Fact button. Click the side of the button with the plus sign on it (or press Command-F4 on your keyboard). You'll be presented with a list of fact types from which you can choose "Occupation."
It's really not obvious as there's not a separator on the button to give a clue that the button has multiple functions (something like on the Timeline button).
----Ben Sayer >@<