Here is how to edit the C152sd.tmd file to correct the syntax error that keeps the airplane from loading in AF2:
use notepad++ or another editor and search for:
<[output_free] [StarterActiveOutput] [ ] [ ]
The location seems to vary with different file versions. emfrat found it at line 1187, I found it at line 1227.
You need to delete the brackets at the far right. Save the file. Restart Aerofly FS2 and go fly the freeware C152.
This error was found by MikeW (emfrat) with hints provided by Jan.