Can't Generate a tier set

I’m trying to set up a large project and have decided to use tier sets. I have turned on tier sets in the preferences and can make tier set labels in the “Edit Tier Set” menu. But when I try to add tiers, the “Add” button is greyed out. I’ve tried it for the current file, another file and domain. I’m just unable to access the tiers that I want to comprise my tier sets. Help!

After selecting the current file, or another file or domain, do you see the tiers listed in the table? Or is that, that doesn’t work? In case you see a list of tiers, selecting at least one tier in the list should activate the Add button, if all’s well (but maybe it is that, that doesn’t work?).