Am pretty new with AFCAD & TTools but have learned a lot recently . . . the following has worked for me:

1) First off, ensure your AI aircraft file is placed within your FS/Aircraft folder - open the aircraft.cfg icon contained within using Notepad, and change the aircraft description to something brief yet concise, such as "Air Canada A340 AI", and save the file.  This description needs to have the quotations around it.  

2) Next, open TTools and then the Aircraft.txt file using Notepad, and assign an AC number to it (just follow the same format used for all other aircraft types here).  I believe there are about 36 AC types by default, you can start from say AC#37 onward, just ensure the aircraft name exactly matches the one used in the description within the AI aircraft.cfg file, with quote marks included (copy & paste recommended), and save the file.

3) Lastly, within TTools, open FlightPlans.txt using Notepad, and insert the commands to create the desired flight using the desired aircraft.  Easier still, I simply copy an already-existant flight plan from an aircraft having similar size/weight/performance parameters, then change the AC# to suit, and adjust times and airport codes where required.  Save the file and run TTools to record the changes.  

Hopefully this makes sense, as I've had a bit of xmas cheer tonight - merry Xmas to everyone here BTW!

Something missing here ... one of the main reasons for doing all the work fruitlessly is forgetting to DECOMPILE TT before any adjustments, and RECOMPILE TT after adjustments or additions are made. exception is the first step, you can add to a/c folder before starting the AI business.
papa-metro usafa 1953-57