Its called gear selection and do to it you need to learn about a very simple file called description.ext
If you are not sure how to use it look at the tutorial mission in Getting Started in the Ed Depot and copy it over to your own mission.
If the whole idea of extra files is new to you look at Macguba's Guide to Mission Editing, also in getting started.
There are also a couple of excellent specific tutes on this file (which does all kind of cool stuff) and you should have a look at those too.