HOw do you find out an aircraft's 4-digit number?

I downloaded a CFS1 mission, Nachtjagd, and am having trouble getting it to run. Message comes up in CFS that there was an error loading the mission and that it may be formatted wrong. A notepad version with the problem appears in and it reads:
[Mission file error]
error=Unit type ID 3001 not found (specified for unit 1012)
title=Nachtjagd 1944 I/NJG1
filename=MISSIONS\Venlo`44
Looking in the mission file itself, unit 1012 is:
id=1012
type_id=3001
name=%name_string.7%
position_lat=
position_lon=
position_alt=
position_pbh=
speed=150
skill=0
level=0
points=0
damage=0
The mission also shows that these aircraft are available:
player_aircraft=3003
allowed_aircraft=3001,3003,3004,3005
How do you know where and when an a/c has been loaded and what its 4-digit number should be?
Thanks.
[Mission file error]
error=Unit type ID 3001 not found (specified for unit 1012)
title=Nachtjagd 1944 I/NJG1
filename=MISSIONS\Venlo`44
Looking in the mission file itself, unit 1012 is:
id=1012
type_id=3001
name=%name_string.7%
position_lat=
position_lon=
position_alt=
position_pbh=
speed=150
skill=0
level=0
points=0
damage=0
The mission also shows that these aircraft are available:
player_aircraft=3003
allowed_aircraft=3001,3003,3004,3005
How do you know where and when an a/c has been loaded and what its 4-digit number should be?
Thanks.