Ozzie,
If you are not moving the iFly will "guess" your departure location by searching for nearby airports (within a mile or two). If none around it will assume your current lat/lon as the departure point. (Note: When in flight it doesn't do this, it will always use your lat/lon as the departure point).
If you want to manually specify the airport you are departing from you can do this by touching the Flight Plan button and modifying your departure point, or you can touch & drag the departure point.
Hope this helps,
Walter