Go to Preferences->Personal Information Options, and add your new PI field. (Aside: You can choose to make it visible and/or required as needed. These settings will apply to all new events you now create. For your existing events, the PI field is added but is NOT made visible or required, unless you manually select those from within a specific event. Basically, TF assumes you want the settings for future events, but that you may or may not want them for existing events.)
Go to Manage Event on any event and go to the PI page. Simply save the page (button on the bottom of the screen). The reason you have to do this is because when you do an export, the query will only join those PI questions that have been associated (e.g. "saved" to an event, even if not visible) with at least one event.