Hi Rogerio,
I’m trying to follow along with you, and I have a PiFace Digital 2 at my desk here (I hope you have the same one). I’m curious what ‘Board Number’ you selected when adding it to Cayenne. When I add mine I’m not able to associate any widgets on it, so trying to figure out what I’m doing wrong.
So that is my caveat that I’m working blind with this response, and I can add more when I get this working 
I believe you should be able to click on the individual widgets on your GPIO list on the left, and it will open a settings dialog where you can change them from the integrated GPIO to the PiFace GPIO. Then you should be able to pick those widgets when building an if/then trigger.