Hey sonic,
I think what you're after would be this :

When you create a flight, or use flight planner, when selecting your airport (starting airport in flight planner),
you should see the drop-down list I've ringed.
This, as you can see, chooses your start location.
You can select many options (including parking spaces) from the list, as long as the airport has them. (eg : Heathrow has hundreds of parking spots/gates, while smaller airstrips may not have any).
Hopefully I've helped,
Cheers,
Carl