I think I've figured it out. You have to be in Edit Mode > Face Mode to make a face active. Also the faces can't be triangulated, if they are you have to select the faces and press Alt J.
I'm trying to map a curved plane, by using the "Follow Active Quads" but ut doesn't work. I get "No active face" . I tried following these tutorials but, it still didn't work. https://wiki.blender.org/index.php/Extensions:2.4/Py/Scripts/UV/UV_Calculate/Follow_active_quads…