If you want to make your model with virual cockpit.

1.Prepare with Gmax exterior model.
2.In "Edit" put "Select all".
3.In "Group" select "Group".
4.Type group name "exterior".
5.Save
6.Prepare with Gmax your interior model.
7.In "Edit" put "Select all".
8.In "Group" write name "interior"
9.Save.
10.Come back to exterior model.
11.In "Edit" select "Marge".
12.Marge interior model.
11.In 'Edit" select "Export".
12.Write name of your aircraft.
13.Save.
14.In "LOD" select " Add..".
15.In "File of types" select "All Files".
16.Find fail with name of your aircraft and "_interior".
17.Change name " (Name of aircraft)_interior.x" !!! IMPORTANT !!! change ".x "
18.In "File of types" select "X Files".
19.Select your interior.
20.Select open.
21.In "Main" select in model type "Aircraft"
22.Select "Start".