I've updated to the latest SDK, but am still having problems.
I've uncommented example 6 in the attached aerofly_fs_2_external_dll_sample.cpp, compiled and place it into the "Documents/Aerofly FS 2/external_dll/" directory. When FS2 launches, I see the "AeroFly External DLL Example" output window show, so I know the DLL is loading, but the aircraft's position and orientation do not change based on the code, they seem to still be driven by the internal dynamics model instead of the DLL.
Side note, The tmcoordinates_PositionFromLatLonAlt function seems to have been removed from the latest SDK, but it still seems necessary to get example 6 working with lat/lon. I tried adding back in the version from aerofly_fs_2_sdk_tools_20180823, and uncommenting the lat/lon related code from example 6, but the behavior is still the same - the aircraft's position/orientation do not seem to be updated by the DLL.