Ken.
You're quite right about that error message. The most common cause is missing, incorrectly installed or incorrectly named  aircraft.

I don't know the tute you mean. Here's how I do it. Open the main CFS folder & find a file named Error.log. Open this in Notepad. Here's an example of an error from a CFS2 campaign I've been testing.

[Mission file error]
error=Unable to init unit 9204
title=KWP06 The Bridges At Wonson-ri
filename=C:\PROGRAM FILES\MICROSOFT GAMES\COMBAT FLIGHT SIMULATOR 2\MISSIONS\TRS_KWP6

In this case you need to find unit 9204 in TRS_KWP6. Find the mission in the Missions or Campaigns folder & open it in Notepad. Type 9204 in the Search/Find feature. It should find an entry like this,

[unit.0]
id=9204
type_id=3902
name=%name_string.0%
allegiance=1
skill=2
aggression=1
payload=2

Now type the type_id= number in the Find box. 3902
You might need to hit F3 a few times to find the entry you need. Here's the problem aircraft.

[unit_type.2]
type_id=3902
unit_family=1
name=KWCFS2_SEA_FuryAI

The name= line usually (but not always) indicates the exact folder name of the aircraft. You might need to repeat this procedure more than once if there's more missing aircraft in the mission. I find using Keith Bedford's Cfsme makes this even easier. http://www.simviation.com/cfs1utilitites2.htm

A common problem with AI Only drones included with some 3rd party missions is missing Panel and/or Sound folders. All aircraft must have these folders to work, even if they're empty folders. This bug is caused by using an early version of WinZip to extract the zipfiles.

Errr,

I must be thick but I can't find error.log...

I've been in cfs main folder, but can't find it.  Any suggestions?  Please?

All the best,
Ken Attwood
(Terminally Confused ??? ??? ???)